When they aren't using your intestines as a holiday home or eating some unsuspecting creature from the inside out, parasites can have some surprising benefits.
Seriously. Meet some of nature's unsung heroes: the parasitic do-gooders.
The body booster
You might imagine having a parasitic worm in your gut to be a fairly traumatic experience, but for unwitting roundworm host Theo Blossom, it was mostly underwhelming. In fact, for the two years he was infected, Theo felt pretty good.
When tapeworms and roundworms set up camp in a body, they give their host's immune system a boost. The body attacks the invader while simultaneously issuing orders to ensure the immune response doesn't get out of hand.
This means that when you are infected with a parasite, your immune system is more active than usual, and better able to cope with other foreign bodies such as pollen or bacteria.
So Theo lived allergy-free and happily unaware of his slippery friend for two years before it emerged, in an admittedly traumatic manner, back into the outside world.
Never mind allergies, scientists are investigating the more dramatic theory that parasitic worms could improve the symptoms of or even cure debilitating diseases in humans.
Some researchers think that particular species of helminth (parasitic worm) have been quietly providing serious health benefits to humans for years.
Ironically, thanks to improvements in hygiene and sanitation, the risk of humans contracting these parasites is much lower and our bodies are no longer reaping those benefits.
One study showed that patients infected with helminths experienced fewer symptoms of multiple sclerosis (MS) than those who were not infected. Other results suggest that certain parasitic species could reduce the symptoms of Irritable Bowel Syndrome (IBS), Crohn's disease, Type 1 diabetes and even arthritis.
Insect pests munch their way through millions of pounds' worth of crops each year. Rather than spraying expensive and damaging pesticides on their fields, some farmers turn to parasitoids for help.
Parasitoid is the name given to insects that behave like parasites but always kill their host. Farmers take advantage of the fact that certain parasitoids naturally prey on particular species of crop pest.
The parasitoid wasp Aphidius ervi, for example, lays its eggs inside unsuspecting aphids. When those eggs hatch, the wasp larvae consume the aphid from the inside out – aphid pest problem solved. This 'biological control' technique is widely used around the globe.
Some parasitoids are out there quietly controlling our pests and getting no credit for it. The gall midge attacks popular UK crop oil seed rape, and until very recently we weren't aware of its natural parasitoid Platygaster subuliformis. This silent wasp assassin keeps gall midge populations down and crops healthy.
Scientists at the Museum are always pleased to receive new parasitoids in the post.
'We get wasps sent in all the time,' says Dr Andrew Polaszek. 'The Museum's huge collection and taxonomic expertise means we can confidently identify the species and therefore the organism(s) it targets. This is really important for identifying whether a biocontrol programme will work.'
The trade of endangered species is a worldwide problem, with thousands of illegal transactions taking place every year. Whether traded as an alligator skin handbag or live snake, these species are protected by the Convention on International Trade in endangered Species of Wild Fauna and Flora (CITES).
When animals on the CITES protection list are seized by the authorities, an investigation begins into where they were bought and who is responsible for breaking the law. Sometimes, parasites are called in as witnesses.
Because some parasites live only in very specific areas, they can help the authorities track down where an animal came from and if the transaction was illegal.
Museum curator Eileen Harris is often asked to identify nematode worms.
‘Once I was sent a parasitic worm that had been found inside someone’s pet chameleon after it died,’ she recalls, ‘The vet doing the autopsy thought it was odd, sent it to us, and we identified it as a species found exclusively in Madagascar. That didn't match up with the dealer's claims about where he'd got it from.'
The parasite helped authorities to prove that the pet had been taken from the wild illegally, despite being sold as a ‘captive bred’ animal. And the worm itself was actually used as evidence to convict the sellers.
The forensic witness
It isn't a pleasant role, but parasites are sometimes called on as expert witnesses in cases of animal or human neglect.
Myiasis is the parasitic infestation of a live mammal by fly larvae (maggots) that grow inside the host, feeding on its tissue. While this is bad news for the sufferer, myiasis-causing larvae can be used in a positive way to bring those responsible for neglect to justice.
Museum scientists work with the police and the RSPCA to identify and age the larvae collected in cases of human or animal neglect.
The larvae can be identified using the Museum’s extensive specimen reference collection. Then, because we know how rapidly different species grow under different rearing temperatures, scientists can age the maggots, revealing a minimum period of neglect.
Museum entomologist Dr Martin Hall says:
‘In several cases involving Museum scientists, our entomology expert witness statements have supported the medical or veterinary reports of neglect and led to successful prosecutions in court.’
So there you have it. While they lead grisly lives and often aren't the most attractive creatures, parasites can actually be good for our health and some even moonlight as crime-fighters.