Infested! Living with Parasites

Parasites. To most people they're simply revolting.

Gut worms, came out of somebody.

-About five or six different worms.

-ALL: Eugh!

Even the word "parasite" is an insult.

How disgusting do you find that on a scale of one to ten?

Disgusting they may be, but I think they're also truly extraordinary.

They have astonishing life cycles

and they are some of the most

successful creatures on the planet.

To find out more about these amazing animals, I'm going

to deliberately infest myself with a range of parasites.

It's like jumping off a cliff. Here we go.

From the largest...

..to the most deadly.

By becoming a host I'm hoping to discover how the human body,

this body, responds to parasite infection.

I'm going to use my infestations to study first-hand

our complex relationship with parasites.

Ahh!

Experience the costs

and potential benefits of being infested.

And even find out what they can tell us about our own origins.

My first, of many, encounters with parasites starts here,

in Nairobi, Kenya.

I'm in search of an extraordinary human parasite.

One which has been

all but eliminated in the developed world.

Now, most tourists come to Kenya in search of wildlife.

Just down the road from here you can go to a safari park where you

will see rhino and giraffes.

I, however, have come in search of something smaller

and much less attractive.

Tapeworm.

The tapeworm is one of nature's most successful parasites.

There are thousands of different species of tapeworm each adapted

to a particular host.

RAT SQUEAKS

The one I'm looking for is taenia saginata.

CATTLE MOO

The beef tapeworm.

It's evolved to live in our guts.

But its peculiar life cycle also involves cattle.

So imagine you're in rural Kenya

and you've got a great big tapeworm inside you.

This tapeworm is producing eggs.

One day you go down to a field and you excrete there -

and you contaminate the grass.

Then along comes a poor, unfortunate cow who eats that grass.

Along with grass the cow eats the tapeworm eggs

which hatch into larvae -

and then enter the cow's bloodstream.

It spreads through the blood to the muscle, perhaps the shoulders

or the tongue and there it forms a really tough cyst.

And it hangs around just waiting for the next stage, which is

for the cow to be eaten by a human.

COW MOOS

The beef tapeworm can only grow to adulthood inside a human

and for that reason it's hard to study.

So I've decided to infest myself,

as my contribution to research into these shy, retiring creatures.

COW MOOS

So, I need to find infected beef.

But in Britain it's extremely unlikely you'll find a contaminated cow,

which is why I've come to Kenya.

But even here tapeworm is getting harder to find, as hygiene improves.

To help me, I've teamed up with parasitology researcher,

Judy Mwangi.

Kind of felt like a good idea when we were planning this,

but now we're actually doing it it seems quite strange.

-Have you ever done anything like this before?

-Oh, no, I haven't.

THEY LAUGH

We've had a tip-off that a meat inspector has found some infected beef.

But to collect it we need to drive into one of Nairobi's tougher neighbourhoods.

It looks quite a rough area out there.

Oh, so what happens now, do we go to the slaughterhouse?

No, we cannot walk to the slaughterhouse,

we have to wait for him to come and bring us the sample.

-OK.

-Yes.

-So he's definitely got a sample, has he?

-He has now.

-OK, all right.

-We've been lucky.

-Lucky!

-Really lucky.

I was kind of hoping you were going to say,

-"No, tragically, they haven't found anything."

-Hmm.

-And will they let me into the slaughterhouse?

-No, they will not.

They're really sensitive about attitudes, anything...

OK, and what about filming, presumably they won't?

That is a definite no-no.

Now we've got infected meat.

We take it to the lab.

So Judy can find the tapeworm cysts.

-So, this is a cyst?

-This is a cyst, yes.

It's not at all what I was expecting.

I was expecting something quite hard.

-Hmm.

-This is quite jelly-like isn't it?

It is jelly-like, because if it's hard it's calcified

so it's not viable.

'I'm not particularly keen on eating raw meat.

'So Judy is cutting away as much tissue as possible,

'just leaving the tapeworm cyst itself.'

It kind of looks like a partly fried egg, doesn't it?

It does, sunny side up.

Inside this cyst is a tiny living tapeworm larva.

It's what I've come all the way to Kenya for

and Judy has found three of them.

Is that the right sort of number?

I think so, one looks a bit dodgy.

So one's a bit dodgy, two of them you think are OK?

-Yes.

-And that kind of just increases the risk...

-..one of them will take?

-Yes, indeed.

-What happens if two of them take?

-The better.

-SHE CHUCKLES

-Better, better for whom?

-Can I...offer you one?

-No, you're all right.

Back at the hotel, the time has come for me to give myself tapeworm.

I'm feeling mildly apprehensive.

I think I'm confident that it should be safe.

It is a very strange thought that there are millions of people

who are infected by parasites who don't want to be

and I'm about to deliberately infect myself.

But I hope it's not something I'm going to regret.

Cheers.

It's like jumping off a cliff. Here we go.

Ah!

There they go.

In just a few seconds, the tapeworm cysts are in my stomach.

That's full of acids that normally help to protect us from disease.

But the tapeworm uses our defences to its advantage.

The acids dissolve the outer case of the cyst, releasing the worm inside.

Which then passes into my intestines looking for a home.

My time in Kenya is over now!

I've done what I needed to do. Now to go home.

If they survive, each cyst will release a tapeworm scolex.

Equipped with four suckers which it will use to latch on to my guts.

Then it starts to grow.

New segments emerging from the scolex.

As it grows the segments will get bigger and more mature.

Whilst new segments appear near the head.

And I'll then use a miniature pill camera

to see what's actually growing inside me.

All this is only possible inside the human body.

This particular tapeworm is so perfectly adapted to the exact

conditions of the human gut that it can't grow anywhere else.

That degree of specialisation may seem bizarre

but it's actually common in parasites.

There's a type of flatworm that can only

live inside the lens of a stickleback's eye.

And a mite that only lives inside a moth's ear.

And perhaps the strangest example of extreme specialisation

turned up here at the Horniman Museum in South London.

One morning, someone brought them something

they'd found in their fish supper.

They basically brought us someone's evening meal on a plate.

So it was a...a cooked fish that the person had started to eat.

Halfway through their meal,

they discovered this inside the fish's mouth.

It was enough to put anyone off their food.

So this is cymothoa exigua, or the fish tongue louse,

and it has a really unusual parasitic lifestyle.

It enters the gills of the fish

and then attaches to the tongue-like structure of the fish

and kind of clings on and pierces the tongue structure

with its big claws

and then basically feeds off the blood of the tongue,

which eventually just becomes quite shrivelled

and ends up being just a little stump that the fish tongue louse

then moves further into the mouth, and basically

starts to live as if it was the tongue of the fish.

So, manipulating food around the mouth.

The fish tongue louse is a particularly extreme example

of a parasite that's adapted its body to fit its host's anatomy.

But we also have parasites that have adapted to fit in with us.

Including...lice.

We have no fewer than three species of lice.

And the story of how these three species evolved can tell us

something remarkable about our own evolution.

The most familiar human louse is the head louse.

To see its unique characteristics I need my second infestation

so I'm off to a de-lousing salon in North London.

-So to remove the head lice... we use a normal Hoover...

-Yep.

..with a specialist attachment, so it's got

a comb on the end of it there and then we collect the lice in there.

Great, OK, well, that's good because I actually want live...

OK!

So, I'm hoping there's going to be a few to... Yeah...

-Well, I'm sure there will be.

-Oh, we shall see, OK.

HOOVER STARTS

Are you at all worried they're going to jump onto you?

They don't jump and they don't fly. The only way they move is by crawling, so...

-Yeah, it makes it...

-If I put my head next to hers they could crawl across.

-Right, then they'd crawl across, yes.

-Or occasionally...

But you're well protected.

-I am well protected, yes.

-I'm less well protected.

'If, unfortunately, you are infested you will probably have

'10 to 20 live lice on your head.

'But there can be hundreds.'

-So, a good collection of head lice here?

-Yeah.

-They're scuttling around.

-There's some young adults,

-there's some babies that have just hatched so...

-They're pretty big.

-Yeah.

-What I'd love you to do then is infect me, please.

-OK.

This is rather unusual, not your normal line of business.

I can do that.

'Head lice can only survive by drinking human blood,

'which they do several times a day.

'To keep these lice alive, I need to feed them on me.'

..this could be my first.

'These little pots should keep them safe while allowing them

'to bite me.'

Thank you very much for infecting me.

You're welcome!

There aren't many people who would be pleased to hear

that their visitor is infested with lice.

-Hi, James.

-Hello.

-I come bearing gifts.

-Oh, excellent.

But James Logan is clearly delighted.

Let's have a look. Oh, that's brilliant.

'James studies lice and other parasites at

'the London School of Hygiene and Tropical Medicine.'

-OK, so here we have one.

-Uh-hm.

-And shall we put it on your arm?

-Why not?

-See if he scurries around?

-Yeah, so I'm...

'Using a handheld microscope, we can study my head louse

'as it settles in.'

But you can really see the blood here, can't you?

-So this one's quite recently fed.

-Yeah.

On your blood.

But you can see it being sort of pumped down here through its gut.

'Like all lice the head louse has retractable mouth parts that

'can puncture my skin.

'But what really surprised me is how active they can be.'

-It's going so fast I can't keep up with it.

-This one is quite active, isn't it?

It certainly is! Look at that, it's gone right to the last hair on your arm!

-Oh, yeah.

-But look at it clinging on to the edge of that hair with...

-Yeah.

-..with two of its claws.

-Look at those claws...

And it's got another claw out like this.

I wonder whether it's just sat waiting for another host to come past.

So that, if I were to rub my head against yours, it would just grab a hold of your hair

with the other claws and it would be across in an instant, would it?

Exactly right, yeah.

It's on your arm, which is not the right place for a head louse.

So it is probably sitting there thinking,

"I'm not in the right place, this isn't the right environment

"and I want to be somewhere where there's more hair."

'Head lice are well adapted to manoeuvring around the human scalp,

'their claws the right size for the hairs on our head.

'But there's another species that makes its home on human hair.

'The pubic louse.

'The good news is that pubic lice are increasingly hard to find.

'When we did finally get hold of these live specimens

'I was out of the country.

'But one of my production team BRAVELY volunteered to let them

'feed on her skin.

'And filmed this unique footage.'

And the first thing that you notice is it looks like a crab

and that's why they get that name, crab lice.

They're very crab-like, their bodies are much more sort of rounded

and they're much smaller as well than head lice.

-What it's designed to do is to live on public hair.

-Hmm.

And pubic hair tends to be spread slightly further apart than

the hair that you get on your head.

And so it's designed like this,

so that it can reach across and grab on to hairs that are further apart.

-And the hooks are a bit bigger as well, you'll notice.

-Hmm.

And that's because pubic hair tends to be thicker than most hair

that you get on your head.

When scientists began to study pubic lice

they discovered something quite startling.

That our pubic lice are closely related to

lice found on another animal.

One of the great apes.

It's thought that they came from gorillas originally and you can see

the sort of divergence between the species that are found on gorillas

today and the species that we get on us around three million years ago.

OK, so does that imply that we had sex, or at least our remote ancestors

had sex with gorillas three million years ago?

I think that's probably quite a difficult one to answer, but it's

probably more likely that we shared a sort of environment with gorillas.

We may have even killed them to eat them

as bush meat for example, as well.

So, certainly we came into contact with gorillas.

Other apes like gorillas and chimpanzees

have a single species of louse.

But when our ancestors lost thick body hair and

gained pubic hair we created two separate islands of hair.

Providing a niche for gorilla lice.

All of which suggests we became relatively hairless

at least three million years ago.

Having two species of lice is remarkable enough but humans

actually have three and the third species also has a secret to reveal.

There's another type of louse called the body louse.

And I've got an example here to show you what an infestation looks like.

And this, remarkably, is highly adapted to clothing.

This was actually... I believe this was from a homeless person.

-Yeah.

-Who had a very heavy infestation.

-Oh.

And I think this...it looks like trousers to me with a seam here.

And this is exactly where they lay their eggs.

-Ah!

-You can see them...

-That is revolting.

-And those are eggs.

-Right.

So that is a massive infestation,

-it makes you feel quite itchy, doesn't it?

-It does.

'The body louse behaves very differently to the head louse.'

So, if we have a look at the lice...

'But if you compare their body shapes you see something significant.'

-OK, so the one on the right, the dark one, is my friend the head louse?

-Yeah.

He's looking rather dead at the moment, I have to say.

Not looking good, and the one on the left, they're the body lice, are they?

That's exactly right. And what you can see is that they look remarkably similar.

And when you look at their DNA

what you find is that they diverged around 100, 200 thousand years ago.

So not actually that long ago in sort of evolutionary terms,

but enough to make them different species. And so that tells us

something about when we as humans started to wear clothing.

So before that, the idea is humans are naked?

Then they start to wear clothes, maybe a head louse

-kind of drops down, thinks it's actually quite a nice place to live?

-Yeah.

And begins to evolve, a whole new species starts to evolve on clothes?

-Oh.

-Exactly right, yeah.

So it's quite incredible to think that you can look at

the louse to find out more about our own ancestry and our own evolution.

So both the lice and the tapeworm have evolved to live on the human body.

Changing their bodies and

behaviour to exploit our anatomy.

But evolutionary pressure works both ways -

over time we have also been shaped by the parasites we carry.

What do you think of those worms?

WOMAN: Horrible.

Gut worms.

Oh!

-WOMAN:

-Would they make you feel bad?

Or would you be happy to have those in your tummy?

-Feel bad.

-A bit bad? Yeah, would it make you feel a bit sick?

'Val Curtis believes that parasites have shaped some

'fundamental human instincts.'

They get hungry, they can smell you.

They can smell your warmth and they come out...

maybe 20 minutes they spend sucking on you and you don't even notice they're there.

And then as soon...

'To demonstrate, we set up a street stall,

'to see what sort of reactions we'll get from parasites.'

Which of these things do you find most disgusting?

Oh!

What about this...?

-What does it do.

-That fly lays eggs on your skin

and then it grows under your skin.

< That's disgusting.

ALL: Eww!

That's how big they are.

Can you imagine having one of those in your leg or in your head?

'Val believes we evolved the disgust response...

'to prevent us

'being infected with disease- causing organisms like parasites.'

What about this?

Most people think of disgust as just being a bad thing,

but actually disgust is incredibly good for you.

Let's just imagine one of your great-great-great ancestors

and that...maybe that great ancestor wasn't squeamish

and maybe that great ancestor went "Oh, oh, my goodness, a poo, look."

HE LAUGHS

-"There's a bit of food left in there, yum, yum."

-That's not, is it, it's plastic...

So maybe, you know, nibble, nibble, nibble on the poo.

Now, your great ancestor who does that,

is he going to find a wife easily?

-LAUGHTER

-Not terribly well, right?

-Yeah.

How about his children and his grandchildren,

are they going to be healthy? No, they're not.

So it's a fair bet that every one of your ancestors had a good,

healthy sense of squeamishness and, as a result,

you have their squeamishness genes.

And it grows into something a bit like that.

'To reduce the risk of infection

'we instinctively find parasites

'and anything to do with them disgusting.'

-The head lice.

-Ew!

How does that make you feel?

-We've just had our hair cut!

-Oh, gosh!

LAUGHTER

'The most likely source of parasitic infections is other human beings.

'For a social species like us, that presents a real dilemma.'

'I want to be your friend, you know.

'I want to learn something from you.'

And I want to exchange with you and all of us want to do that.

That's the human way of life.

We're a deeply social species, we co-operate, we exchange.

But you're at the same time a bag of parasites.

Yeah.

You're absolutely stuffed full of parasites.

So I don't want to get too close to you.

I don't want to be within spitting distance of you.

And in fact, if you look at the way people

behave around the world, we don't sit too close to each other.

We're very careful about wearing clean clothes.

We're very careful about not being smelly and not being...

-not sharing our bodily emanations.

-Yeah.

-That's good manners.

So you think that manners are a product of parasites?

Parasites teach us manners

and manners are fundamental to what make us human.

Hurrah for the parasites?

Oddly, my reaction to having a tapeworm infection is more

curiosity than disgust.

To find out more I met up with the UK's

leading tapeworm expert.

OK, it's about a week now.

It's going to be about that long?

I should say so, yes. I'm taking that as

a calculation

that after 10 weeks I know it'll be 3m.

But we don't really know now, of course.

We're just assuming the three cysts you took were viable,

and they looked viable, at least two of them, I think.

-How would you tell, in any way?

-I don't think you will...

No, I think...

if you didn't know that you were infected, you wouldn't know.

No. I have these moments when I think...

I've got a little gut rot

but then I think...

The fact that you know you're infected

means that you might feel like you've a bit more

of a craving for beer or for carb...or for chocolate.

Or something that you like, perhaps chocolate?

No, absolutely, that's what I said to my wife.

And people do report that, you know, they do eat more.

A tapeworm has no digestive system of its own.

No mouth, no stomach.

No anus.

Instead...

it absorbs the food I've already digested.

Its skin is very similar to the inner surface of my intestines.

With lots of tiny bumps called villi.

These increase the surface area of the tapeworm

so it can absorb nutrients

directly through its skin.

So, what should I feed it?

It can take up carbohydrates

and glycoproteins very effectively.

-It likes them?

-It likes them.

'Anything rich in carbs will be good for the worm.'

In fact that does remind me of the type of tapeworm that you

might have.

-Yes, the proportions are fairly accurate actually.

-Yeah.

That's going to be about the width of the worm after about 10 weeks.

Yeah.

And certainly, if it's left longer than that,

going into months it will get about twice the width.

So the worm likes carbohydrate?

Well, I think it would, yes.

HE SLURPS

Certainly that's good for a growing tapeworm, Michael.

HE LAUGHS

If you have a bowl of that a day.

-Yes, I'll give it a go.

-It'll grow.

-Good and strong.

-Probably to good proportions, yeah, yeah.

Head lice are rarely more than an irritation.

And so far my tapeworm is proving very benign.

It's a strategy that makes sense.

By causing minimal harm the parasites help to ensure

their host's survival.

But not all parasites take this approach.

An example of a lethal parasite is this one.

It's a microscopic, single-celled parasite...

..called plasmodium.

It causes malaria, a disease that kills more than

half a million people a year.

I want to find out why it is so deadly.

So, I've come to the National Institute of Medical Research,

where they will infect my blood with plasmodium.

-So, we've got a sample of my blood here looking...

-Yes.

..nice and red, healthy?

Yes, yes, yes, and warm, straight out of your body.

-MICHAEL LAUGHS

-Hi, there.

-Hi.

-I won't shake hands.

-LAUGHTER Thanks very much, Fiona.

-OK.

-Well, I guess we're not allowed in that room.

-Oh, that's right, yes.

So, what's Fiona up to at the moment, then?

So, what's she going to do now is she's going to take

a sample of plasmodium falciparum.

So this is the most dangerous form of the parasite that causes

most deaths in, for example, Sub-Saharan Africa.

We've grown this in the lab.

We can culture it continuously in human red cells

and she's now going to take a sample of those parasites and inoculate them into your blood.

Right, and I must admit, out of all the diseases, malaria is high on the ones I want to avoid!

Well, you should try to avoid it if you can.

To see the malaria parasite multiplying in my blood

they're using a new photographic time-lapse technique.

Although I have treated people with malaria I have never seen

the malaria parasite in action before.

Very intrigued to see what happens to my blood,

but what is chilling is thinking that out there in the world

mosquitoes are infecting children.

And a thousand children every day are dying.

And that's why this sort of work is so important.

Here it is, my blood infected with the deadly malaria parasite.

So, the parasite goes through this life cycle in the infected

red cell, it infects...

'Once the infection is complete, Mike Blackman shows me

'the finished movie which reveals how the parasite kills its host.'

-The little white blobs here, they are the parasite, is that right?

-They are the parasite, yes.

So a single merozoite, this invasive form of the parasite.

Binds to a red cell, invades it, grows within it.

Digests the haemoglobin of the red cell.

This is the red protein that is used to carry oxygen by your red cells.

-And then eventually forms around about 16 to 32 daughter merozoites. Right.

-Yes.

-So they're... Oh, wow, look at that one go.

-Yes.

-That one just exploded, didn't it?

-That's right... So that...

-And suddenly you're seeing lots and lots...

-Oh, wow, they're really going.

-Yes, yeah.

The whole thing is destroyed in a single, very rapid process.

The merozoites are released and they immediately invade a new cell and these things just...

I had no idea it was going to be that violent.

I mean, that was utterly destroyed.

'Each explosion obliterates one of my oxygen-carrying red blood cells

'and releases new parasites into my blood to infect yet more cells.'

And suddenly you go on from a situation where

there are relatively few, but they're everywhere, aren't they? They're just swarming.

I mean, you do appreciate just what a terrible thing they're doing inside you.

Yeah, if this were going on inside you, you'd be in a pretty poor way.

'You soon become anaemic from lack of blood cells

'and the debris from all those shattered cells

'can block your blood vessels, which, in turn, can be fatal.'

So our interest here is exactly what...how this goes on.

How does the parasite actually do that?

We don't really know.

The parasite is very, very good, very smart.

It's evolved with its human hosts for a long time.

MOSQUITOES BUZZ

For all parasites,

getting into a new host is essential for their long-term survival.

That's particularly important for parasites that kill their hosts.

For plasmodium the key to infecting a new human host is this -

the mosquito.

New research has revealed that once the malaria parasite

infects a mosquito

it actually sharpens its sense of smell.

Making the mosquito more likely to seek out and bite a human being.

And as it sucks our blood it injects the parasite into us.

But there are other parasites that use even more

extraordinary strategies for getting into a new host.

Like this parasite - a kind of flatworm.

Although it spends the first part of its life cycle in a snail,

it needs to infect a bird in order to reproduce.

To make sure the snail becomes bird food, it crawls into the

snail's eye and the eye swells up to resemble a tasty caterpillar.

What's more, the worm wriggles to put on

a literally eye-catching show.

It also changes the snail's behaviour.

Usually, a snail hides in dark places,

but an infected snail will happily come out into the light.

This massively increases its chance of being eaten by a bird.

Bad news for the snail, but great for the parasite.

I do find it particularly chilling, the way that the snail parasite

hijacks the brain of its host,

and makes it give up its life for the sake of the parasite.

But this is not an isolated example.

There are other parasites that adopt similar strategies.

This is toxoplasma gondii.

It's a microscopic parasite that looks like a wriggling comma,

and it's possibly the most successful parasite on Earth.

Its life always starts in a cat, where the parasite breeds.

An infected cat passes oocysts, tiny capsules containing the parasites.

The oocysts can survive on the ground for months,

waiting to be eaten by a rat or mouse, which then becomes a carrier.

But for the life cycle to complete, the parasite has to get

back into a cat, which means the rat has to be caught and eaten by a cat.

So toxoplasma has to encourage its mouse host to commit suicide.

A few years ago, Joanne Webster set up a series of elegant experiments

to see how toxoplasma might go about altering a rodent's behaviour,

making it more likely to get eaten.

She placed infected and uninfected rats in a chamber,

where she had liberally doused one corner with cat urine.

And we simply plop the rat in and let them

tootle about over each four hour, ten hour night,

and simply watch where they went.

Alice here is an uninfected female.

She smelt the cat area and she shot off here.

-She seems to be avoiding the cat-smelling one.

-Absolutely.

-She's hanging around in different areas.

-Yeah.

'Putting in a rat infected with toxoplasma

'produced a very different result.'

This is Felix here.

He's infected, and he's actually in the cat zone.

Which is actually not what you'd expect.

A normal mouse or a normal rat would run.

-Smell it and absolutely hide.

-But he seems to be just enjoying it.

He's out, he's active, he's tootling about.

So presumably, if you're the parasite what you want, you want

the rat, Felix the rat, to hang around near where cats are,

-because you want to be eaten.

-Yes.

Joanne had shown that infected rats are attracted to cat smells.

They're also generally more fearless and have slower reaction times.

All these factors made them far more likely to become cat victims.

What is rather disturbing is that this parasite, toxoplasma,

also infects us.

Around a quarter of the British population are infected

without knowing it.

You can catch it from cat litter.

Or infected soil.

Or from eating undercooked meat from an animal that was itself infected.

Since toxoplasma can manipulate rat brains,

can the parasite also change our behaviour?

There's intriguing evidence that it can.

Scientists have analysed blood samples from people involved in

traffic accidents, and they found they are more than twice as likely

to be infected with toxoplasma than the average.

The claim is that infected people are slower to react

and take greater risks.

Just like infected rats.

I'm a keen cyclist,

and I admit I've had more than my fair share of accidents.

A while ago I was in collision with a white van and I got knocked out.

More recently, I hit the kerb while cycling.

Came off, fractured my ribs and also broke my elbow.

Now, my family thinks it's because I'm careless, reckless.

But could it be it's because I'm infected with toxoplasma?

I've had a blood test to find out, and Joanne has the results.

What do you feel you are?

I would be, I guess, surprised if I wasn't,

in the sense that I am aware that sometimes I do remarkably

stupid things and in retrospect I wonder why I've done them.

-So you could have a few parasites.

-I wonder if there are a few.

I am dying to find out, I must admit.

And you are negative.

I am negative. OK, so I can't blame the parasite for my risky behaviour,

I just am. It's quite scary isn't it,

the thought of a parasite manipulating your behaviour?

Absolutely, and I think the fact that it almost brings on the concept

of free will, because how much of your behaviour are you expressing

is yours and how much is it the parasite within you?

Does toxoplasma change human behaviour?

At the moment, there's no direct evidence.

If there is an effect it will be, in the vast majority of cases,

very subtle.

But the fact that it can alter a rat's behaviour is nothing

short of remarkable.

It shows just what extraordinary abilities evolution

has equipped parasites with to ensure they spread to a new host.

Tapeworm may not go in for mind control...

..but they do have some ingenious tricks.

At two weeks old, my tapeworm is still a harmless youngster.

But when it reaches maturity,

it will start doing something rather unpleasant.

That is the adult tapeworm, and this tapeworm is exactly ten weeks old.

We know that because it was from a previous volunteer.

By ten weeks, the mature segments are full of eggs, and they break off

from the tapeworm and emerge from your body...

..ready to infect a cow.

So it comes out of your bottom?

And it can actually crawl out?

Unfortunately, yes. Psychologically, that upsets quite a few people

that are infected.

Yeah, it can come out under its own steam, as it were.

So, once the segment is in the rectum, it'll move around and that

causes a sort of a strange fluttering sensation, and then

it will move through the rectum, through the anus, it'll crawl around

between the buttocks, down the legs and out onto the floor.

-Right.

-Can be embarrassing.

It might surprise my wife or friends.

I think it probably would, yes.

Often, the first time someone realises they are infected

is when a segment crawls out.

One of the reasons I'm doing this self experiment is to help

Phil change that.

I'm going to be sending you some sort of faeces samples

on a weekly basis. What do you want them for?

Well, that's to... For us to help develop or refine a diagnostic test.

Even though this worm is very big, people that are infected

aren't always aware they're infected, especially early on in the

infection, and we can actually take a faecal sample, a stool sample,

we can test it for presence of tapeworm carbohydrates.

We can also test it for tapeworm DNA, and obviously,

if we find that, it suggests that the carrier

does have a tapeworm infection.

Phil's research is important because tapeworm can be very dangerous.

Beef tapeworm like mine are relatively benign.

But other types, like the pork tapeworm,

can form cysts in the brain which can kill.

So far, I've encountered nothing

but the downside of being infected by a parasite.

Yet, as we've seen,

parasites come equipped with a remarkable range of skills,

and sometimes we can use their skills to our advantage.

This is a leech. For thousands of years, they were used to treat

everything from skin diseases to fevers.

Those cures almost certainly did more harm than good.

But in the right hands, the leech can be a useful surgical assistant.

Ian Whitaker is a reconstructive surgeon who is pioneering

the modern use of leeches.

I normally use them on extremities. For example, a finger,

or in rare instances, a nose or an ear.

You're re-attaching the end of a finger or something?

Yes, that's right.

It is technically possible, with microsurgery, to attach the artery,

so you've got blood flowing in,

but the blood flowing out via the veins -

-it's much more difficult.

-Things start to swell up?

Yes, that's exactly right, that's the basis of it.

And the leeches are quite good at controlling the flow, are they?

Yes, it's almost as if they're custom-made.

You know, they remove a fairly predictable amount of blood

and it's self-contained.

They promote bleeding after their removal as well.

'It's the ability to remove blood that makes the leech

'so useful in surgery. But how much blood can a leech consume?

'Well, there's one way to find out.

'First, we have to see how much the hungry leech weighs.'

-0.4.

-That's perfect.

-OK.

-So shall we put it on?

-Are you ready?

-I'm ready, yeah.

He only needs to feed once a year.

It takes about three months to digest a blood meal.

I can feel it, yeah. I can definitely something going on there.

At the moment, it's attached by its head end where the jaws

and teeth would have made a hole.

Here, you can see how the leech's triple-jawed mouth is able to

cut through a membrane. That's what it's doing to my skin.

Once it's made a hole, it can start feeding on my blood.

It sucks by contracting its muscles

in a rhythmic movement called peristalsis.

Its digestive tract visible here in red can expand

to hold huge quantities of blood.

And its saliva contains proteins that help the blood keep flowing.

And it will be injecting some fairly ingenious things,

hirudin is probably the most important,

which will prevent it clotting, so it can feed more efficiently.

It's the most potent anticoagulant known to man.

How long will I go on bleeding for?

I think the average is about 12 hours.

In some instances, it can go up to 48 and 72 in extreme circumstances.

Oh, God, it's moved. It's come off. Oh, there we go.

Shall we see how heavy it is?

Hey, blimey, so it's now seven or eight times heavier.

That is phenomenal. On you that would be...?

That's equivalent of me... 115 to 120 stone after one meal.

By helping blood to flow freely into a newly reattached tissue,

the leech can save parts of the body that would otherwise die.

It's a very upsetting injury to lose a finger or a thumb

and leeches are literally the answer in some cases

when we can't get any other way to remove the blood,

so they literally will save people's careers.

'The growing use of leeches in surgery has led to unusual

'business opportunities...

'..leech breeding.

'I've come to visit the UK's only leech farm.'

So how many leeches do you have here?

It's about 35,000 at the moment and it goes up to about 70,000.

I wouldn't have imagined there was a world demand for 70,000 leeches.

'So how do you go about farming leeches?'

These are our breeding chambers.

The leeches in the wild they would breed in water

-and they come up into land to lay their eggs.

-OK.

-So this is kind of like a swamp?

-Yes, like a swamp.

You can see there's a freshly-laid cocoon there.

'Each cocoon contains 25 tiny baby leeches.'

Oh, they're bit nasty, aren't they?

-I know, you never tire of the miracle of birth.

-Yeah.

'Most of the leeches here are of the species called Hirudo verbana...

'..but they also have another significantly larger

'species of leech that normally feeds

'on Southeast Asian water buffalo.'

We tend to use this species mainly for veterinary use.

Will it bite? Ow! That's unpleasant! That is so unpleasant!

That really freaked me in ways I wasn't expecting. Eugh!

Funnily enough, I found that buffalo leech much more disgusting

than swallowing the tapeworm cyst.

It's now over three weeks since I did so and the worm should be

at least a foot long, but it still isn't causing me any problems.

I haven't noticed any stomach upsets,

perhaps a little bit more craving for sugar.

Definitely feel the urge for chocolate and biscuits

but that's also completely normal.

So absolutely no side effects.

Apart from dreaming on one occasion I had all these little things

coming out of my bottom. Nocturnal emissions!

But when I woke up I was relieved they weren't there.

A bacterial gut infection will often cause diarrhoea and stomach pains.

And yet I feel nothing.

To find out why, I met up with Helena Helmby,

who studies how worms interact with our immune system.

You have a very fine collection of worms here.

Some of them I recognise.

Tapeworm... This one I'm less familiar with.

This is Ascaris, the large roundworm.

The thing that puzzles me

is how do these worms evade the human immune system,

because some of them are huge?

Yes. And we have a very sophisticated immune system

which is constantly on watch.

Guarding against any microorganism 24/7.

But these large parasites have developed a very sophisticated way

of dealing with our immune system,

because they have evolved with us for thousands if not millions of years.

The worms have evolved mechanisms to dampen down our immune responses.

By secreting compounds that manipulate our immune system.

That enables the worms to survive, but may also have benefits for us.

The worms are allowed to stay because the immune system realises

that the attack to kill them would be far too dangerous for the host.

Obviously you don't want a massive inflammatory response in your intestine

because that would cause a lot of damage to your intestine,

and that would be dangerous for you as a host.

So there's a sort of uneasy truce

between the worm and the host in this case.

It is extraordinary, isn't it?

-Yes, absolutely fantastic!

-You're a fan?

Yes, I love them!

So our bodies have evolved a kind of ceasefire with the worms.

But in a modern, technological society,

most people spend their lives worm-free.

Some scientists believe this may have contributed

to the rise of allergies.

It's known as the hygiene hypothesis.

So you buy the idea that one of the reasons we have this huge surge

in allergies is because we're just not exposed to as many worms

or microorganisms as we used to be?

It's quite possible, because that particular arm of the immune system

that reacts when we get a strong allergic reaction

to grass pollen or something like that,

is also the same arm of immune response

that we use to fight parasites.

And if we are now not having these parasites, perhaps that part

of the immune system doesn't really know what to do with itself

and it goes off and reacts badly to innocuous allergens

such as grass pollen or cat dandruff, or whatever it is you're allergic to.

So perhaps having a worm inside you

will help control your immune system and keep it in balance.

It's not proven but the evidence is intriguing.

There's no doubt that there's been a huge rise in allergies and so-called

autoimmune diseases as we've got richer and more hygienic.

And rates of these diseases are highest in countries that

don't have lots of parasitic worms.

But what is controversial is that some people are now using worms

to try and treat their diseases.

Daniel Heyman has Crohn's.

It's a debilitating condition in which the immune system

in the gut starts attacking the body's own tissues.

I was in Australia, travelling with my parents.

I was very ill the whole time I was there but it wasn't diagnosed

until I got back.

By which point I'd lost several stone in weight

and was bleeding internally. It was pretty serious.

It took me a while to kind of work out what I could eat

-and what I couldn't.

-Curry was not...

Curry wasn't on the menu at all, no, I was limited to fish, rice,

very plain foods, certain fruits, bananas I could eat.

I had also a problem with my lips, they started crusting up,

-which is another symptom of Crohn's disease.

-Really?

The immune system sort of attacks the whole digestive tract.

Fed up with the side-effects from the drugs,

Daniel turned to an unconventional treatment.

Hookworm.

These little bloodsucking parasitic worms live in the intestines.

Daniel hoped they would suppress his gut's immune system

for their own survival.

And by doing so reduce his symptoms.

Daniel ordered a tube of live hookworm on the internet

and placed them on his skin, where they burrowed in,

eventually finding their way into his gut.

That was two years ago.

How quickly did you notice changes?

It was a gradual process over about a year.

I realised I didn't have to use the cream on my lips.

That was a real realisation that some kind of change had taken place.

Were you spooked by the idea of having a parasite inside you?

I wasn't, you know. I think human beings are designed

to have some kind of parasites in them.

It's part of modern life. We're so clean that we don't have any,

but if you were living in a natural environment,

in a situation that our bodies have evolved to live in,

then you would have some kind of parasites going through you.

I'm just going to try this.

-Do you have any fear of food at the moment?

-No, I'm really free.

Thanks to the worms.

THEY LAUGH

I am basically free to eat as anyone else would.

Daniel certainly makes a really compelling case for hookworm.

I just wish there was some proper clinical trials

that would support what he's doing,

because there is always the risk that he happens to be exceptional,

and also a risk that the hookworm might make your symptoms worse.

But I'm very pleased it's worked for him.

Helena Helmby, like most scientists,

finds the idea of self infection very troubling.

Scientists have worked for 100 years to eradicate these diseases

and now we want to start reintroducing them into people again!

These worms live in the gut and suck blood from your intestinal mucosa.

They move around a lot as they do that,

so they cause a lot of bleeding.

If you have a high dose of hookworm in your gut,

you will become anaemic quite quickly.

Rather than infecting people with human parasites,

we should looking into purifying these molecules

that the worms produce and turn them into drugs,

turn them into worm pills.

The hope is that these studies will lead to new treatments

for some of modern life's most debilitating conditions.

My tapeworm, if it's there,

has certainly been living in harmony with my body.

Six weeks on and I still can't feel a thing.

Of course, there is the possibility that the worm isn't there -

that none of the three cysts survived.

The best way to find out is using this.

A miniature camera, which you swallow.

OK. And I'm just going to place this on the front of your abdomen,

and now I'm going to attach the data recorder,

so that just plugs into the side here.

So in your own time, if you want to swallow the capsule.

-Never previously used. I'm assuming?

-Never previously used!

Down the hatch.

-Right, I think that's done.

-Gone.

-And there's your tummy.

-There's my stomach. That is weird, isn't it?

There you go.

Live from my stomach.

Somewhere in the small intestine is the worm lurking, we hope.

It's going to be quite strange to come face to face with it,

so to speak.

Before I can see the worm, I need to wait for the pill camera

to pass out of my stomach and into my guts.

So let's see where it's got to.

It's still in the stomach.

So it's hanging around a bit.

It's not until the evening that the pill

finally moves into my intestines.

Ah! That... That is it.

That is the tapeworm.

You can see it.

Oh, God, that is horrible.

You can just see its tail and its segments, waving around.

I don't know if I'm absolutely disgusted, or...

I'm actually quite excited. Wow.

Because it would have been hugely disappointing to have gone

all the way to Kenya, come back and seen nothing.

What's amazing is

I've not experienced anything and yet you can see it there...

That is so weird!

Right. I wonder if I'm going to sleep tonight.

I'm dying to go back and show it to my wife, see what she makes of it.

This footage provides scientists with a rare chance to see

a live worm in its natural habitat.

So we sent the complete video to a team at Salford University.

Just at the back there you can just see the thinnest

part of the worm, this is the front end.

The segments are getting more mature as you're moving down

the length of the intestine - they're getting bigger,

becoming sexually mature.

Probably they're starting to get fertilisation taking place,

so the worm is actually starting to reproduce at this stage.

Can you tell at this point how big it is? I had no idea of the scale.

By the looks of this it's certainly over a metre in length.

So a metre?

Yeah, certainly a couple of feet, probably three to four feet.

Right. One of those.

So this is the end of that worm.

So it's worthwhile having a look further down the intestine

-to see what's there.

-Right.

-Oh...

-Ah!

-MICHAEL LAUGHS

That's another worm, yeah?

You have more than one worm, definitely.

Right. And so will it be competing with the other worm for nutrition?

They are definitely in a different place,

and it's quite possible that they are trying to avoid

being in the same place so they're not competing with each other.

There is a lot of nutrients in the intestine

but the worms may well space themselves out deliberately

so that they can get the most out of what you've been eating.

I'm quietly thrilled.

Slightly disgusted but quietly thrilled, I must admit.

-I'll stop there.

-OK.

-And guess what, you've got three worms.

-Whoa! Blimey.

This is the very front end, the bit that attaches onto the intestine.

There are actually four muscular suckers around that sort of head

and those are what's latching onto

-the wall of your intestine, keeping the worm in place.

-Right.

Wow. 100% hit rate.

I have to say, a very successful experiment.

With three worms inside me,

I definitely don't want the segments crawling out.

Time to treat my infection.

Generally with worm infections a single dose of a medication does

the job, so you're going to take those four tablets with some water.

-All four?

-All four of them.

OK, down the hatch. Goodbye, worm.

The pills kill the worms.

Which are then absorbed by my gut.

A suitably ironic end -

parasites digested by their host.

It has been absolutely eye-opening

spending quality time with parasites,

and discovering their ingenious strategies,

from the relatively benign beef tapeworm

to the frequently lethal malaria parasite.

From the subtle manipulative skills of toxoplasma

to the full-on assault of the leech.

But above all

I have learned enormous respect for these extraordinary creatures,

and in future I will think twice

before using the word "parasite" simply as a form of abuse.