When Tanya Andrews returned from a recent family holiday in Costa Rica, she had no idea she had brought back a gruesome souvenir.
Tapeworms can grow to several metres
A month later she developed an extremely painful lump on her head.
At first, she thought she had an abscess, but then it wriggled.
At the Hospital for Tropical Diseases they recognised the problem straight away - it was the living maggot larva of a botfly.
While Tanya was enjoying her holiday a mosquito had delivered a tiny botfly egg onto the surface of her scalp.
The egg hatched into a maggot and burrowed deep inside. Incredibly, this happens to thousands of people every year.
As we travel to ever more exotic holiday destinations, we are at the mercy of a whole range of bizarre parasites just waiting to colonise us.
Soon after travel writer, Broughton Coburn, returned from Nepal he began to experience regular, inexplicable nosebleeds.
The Botfly larva pulled from Tanya's head
They continued for three weeks until an embarrassing encounter in a teashop made him realise that something was seriously wrong.
As he was being served, the waiter took one look at him and fled in horror.
Broughton chased him down the street urging him to tell him what was wrong. But the boy would only point, wordlessly, at his nose.
Broughton returned home and sat in trepidation in front of a mirror.
His patience was rewarded when a brown worm-like creature emerged from his right nostril and looked around.
"I swear it had two beady eyes on it. And it came out two or three inches, looked around and then retracted. I thought it was a dream, a vision of some sort."
In shock, Broughton rushed off to his doctor who tried to remove the mysterious creature.
A thirsty aquatic leech
But it wasn't going to give up its home easily.
"He had this thing pulled out eight or ten inches and I'm looking at it cross-eyed down the end of my nose, and he's looking at it, he has a look of absolute horror on his face. And the thing came off. And there was this leech."
Broughton had been invaded by an aquatic leech. It made its move while he was drinking from a mountain stream.
These thirsty bloodsuckers can drink three times their bodyweight at each feed and inject an anaesthetic so their victim feels nothing.
But not all parasites are unwanted and uninvited.
As part of a University of Salford experiment to develop a diagnostic test for beef tapeworm, biologist Mike Leahy volunteered to grow this gruesome parasite inside his own gut.
Mike swallowed the immature tapeworm cyst with a glass of red wine and the worm started to grow at an initial rate of four centimetres a week.
Twelve weeks later he had to call a halt to the unusual experiment because he was getting married!
After a dose of anti-worm pill Mike passed out an intact tapeworm three metres long.
The tapeworm cyst that Dr Leahy ate
Disgusted? Well according to Dr Val Curtis, an expert on hygiene, this reaction is a natural survival mechanism.
"In the same way that you have an immune system which helps to protect you from parasites we also have a behavioural system.
"When you feel the emotion of disgust it is a driver of your behaviour to make you keep away from or drop the thing that might be about to make you sick."
And, it seems, we need all the protection we can get. Every living thing has at least one parasite and many creatures, including humans, have far more.
In fact, parasites make up the majority of species on Earth.
Bodysnatchers was transmitted on BBC One on Wednesday, 26 November, 2003.