There's something about brain-altering parasites that is just creepy. This is doubly true when the parasite makes the host attempt suicide - which is just what Spinochordodes tellinii, the hairworm, is best at.
Hairworms are free-living aquatic organisms as adults, who, as nematodes, eek out an existence in the mud. As a larvae, however, they're nasty little buggers. Hairworms infect Orthopteran insects (aka grasshoppers and crickets). The adults reproduce in the water, and produce little larvae which are eaten by their unlucky hosts. By the time the little sucker is full grown, it can be four times the size of its host - which, I imagine, isn't exactly comfortable for the cricket, especially since the parasite has grown so big feeding on its tissues. But if that isn't bad enough, the hairworm needs to return to water so it can go back to being a boring, mud-loving nematode - and that's where the mind control steps in.
How exactly the worm control the cricket's brain is unknown, but theories suggest that the parasite has developed a way to mimic natural chemical signals in the bug's brain. Analysis of the compounds of infested brains reveals heightened levels of neurotransmitters and chemicals responsible for movement and orientation, particularly with relation to gravity. These proteins are similar to the ones produced by the insect, but not naturally occurring, suggesting that the parasite is able to produce and excrete its own chemical signals to screw with the cricket's brain.
The end result of which, much to the parasite's joy, is that the cricket seeks out water and hops right in. More often than not, the act is fatal - crickets are terrible swimmers, and most who leap into the cool depths drown. This assisted suicide gives the parasite the chance to break free and wiggle its way easily down to the mud, where it continues its life cycle. Of course, that is, if the thrashing cricket doesn't attract a little attention itself, first. A study in Nature found that, although the parasite does a good little bit of mind voo-doo to get to the water, its suicidal host sometimes gets it in a bit of a pickle - and the worm doesn't always get away cleanly. Predators often find the unlucky cricket before the worm has escaped. What does the trapped worm do then? It turns out the parasite has developed a way of escaping from its hosts' predators mouths and digestive tracts, which is quite an impressive evolutionary feat, as digestive tracts are rarely a nice place to visit.
It makes you wonder whether similar chemicals might be present in our brains, and whether some parasite will find a way to make good use of them. Scientists have already found that the brains of suicidal people show distinct differences in gene expression, though its unclear if these variations are inherited or environmentally induced. Perhaps The Happening could actually happen... *eerie sci-fi music*