There are a whole host of parasites (pun accidental but approved in retrospect) that can manipulate the minds of the animals they occupy, forcing them to carry out all kinds of strange behaviors right out of a horror movie — one of the really bad ones, like Human Centipede but with actual centipedes.
Take for example horsehair worms, named for their resemblance to horsehair. If they were named for what they do, they'd have to be named something like "suicide worm", which is probably why they went for "horsehair". These parasites begin life as 0.01-inch larvae, emerging from eggs wrapped around water plants. The larvae make their way to the edge of the water, where they attach to grass or other vegetation, at which point they're immediately eaten by crickets. Don't feel bad for them, this is all part of their plan.
Now inside the crickets, the worms bore their way through the wall of their host's gut and into their body cavity, where they can grow up to 2 meters (6.5 feet) in length, contorting and writhing and digesting their host from inside the cavity. Believe it or not, this is still not the horrifying part. In order to be able to breed and continue their horrifying circle of life, they need to get back to water. Usually, crickets stay away from water, due to not wanting to drown to death. When they have these parasites, however, their fear of death seems to disappear, replaced with an irresistible urge to seek out water and leap into it, drowning themselves very quickly. The worms then emerge from their host in order to breed. It's the CIIIIIIIIIIIIIRCLE OF LIIIIIIIIIIIIIFE.
So far, it's unknown the precise mechanism by which the parasite turns their host into water-seeking zombies, but we do know that they appear to be producing large amounts of neurotransmitters while they are inside the crickets.
Then, there's this little monster.
The fungus infects houseflies and fruit flies, as well as other species. After a fly has picked up the fungus' spores, it behaves as it normally would for a few days, though those days are now numbered. The fungus is growing inside it, feeding on its innards, and taking over its nervous system.
Four or five days after the spores made their way into the body, the fly begins to behave oddly. It climbs to a high spot in the vicinity, in what is called "summitting" behavior, gluing itself in place. Once perched on top, it begins to twitch.
The fly finally extends its wings upwards, before dying and remaining in this odd position. This is when the fungus begins to make its way out of the fly's abdomen. For some reason, the odd fungus, more than living up to its moniker, only kills at dusk.
We're still not at the worst bit. By getting up high, the spores can spread, but the wings being in an upright position also has its advantage. Male flies will mate with the dead body, getting themselves infested in the process.
“I think the fat females are especially attractive for the males,” retired UC Riverside entomologist Brad Mullens told KQED's Deep Look, describing the whole ordeal as a nightmare for the flies. "If their little brains could comprehend it, they would live in fear."