Seven Zombie Parasites

Strepsipterans
Strepsipteron's are in the insect family, the males are gnat sized flying critters with huge eyes, fine senses, and the lifespan of a few hours. There sole purpose, like all males, is to mate with a female. The female Strepsipteran is a limbless, eyeless, bag-like parasite that lives in the body of another insect, such as a fly, bee, or even a preying mantis, with only her head sticking out of the host's body to breathe. To find herself a mate, the parasite will release her mating pheromones on the wind and force her insect host to wait patiently in an obvious and convenient location, such as the tip of a long leaf or twig. Try to imaging having to stand around for hours while the face on your back flirts with strange, tiny men. Days later, she'll upchuck a bunch of live larvae on the next flower you visit, the perfect place to infect more insects.

Gordian Worm
Once known as "horse hair" worms since they would appear mysteriously in horse troughs, Gordian worms spend their parasitic larval stage within the bodies of insects, especially crickets, but spend their non-parasitic adult stage in water. Crickets aren't known for their swimming ability, but try telling that to a parasitic nematode. When it's time for adulthood, the worm compels its cricket to seek out the nearest body of water and dive right in. The confused cricket usually drowns, while the worm wriggles free to find itself a mate.

Sacculina
The sacculina is a type of barnacle, a crustacean just like its crab hosts. The female begins her life in a microscopic, shrimplike swimming stage, but will discard more than 90% of her body when she locates a crab, reducing down to a blob of raw cells which grow "roots" throughout the host and eventually create a small opening for the male scacculina to enter and mate with her. If the host crab is female, it gets tricked by the parasite into carrying, nurturing, and spreading larval sacculina as if they were its own little crablings. Even if the host crab is male, the sacculina transforms its body and mind function just like a female.

Ribeiroia
While this tapeworm relative doesn't pull any fancy mind control, it does perpetuate itself by transforming its host into a monster. The ribeiroia usually prey on tadpoles and will tamper with the tadpole's development into a frog to create horrific deformities, such as multiple legs and arms at awkward, random angles, making it extremely difficult for them to swim or hop. The only purpose of this extreme transformation is to get the frog caught and eaten by a predatory waterbird for a free flight to the next pond.

Pseudacteon
Though related to the harmless fruit flies breeding in the world's neglected fruit bowls, Pseudacteon flies have a far more sinister appetite. The female lays her eggs in the body of a living ant, and the tiny maggot will eventually more into the ant's head to devour its brain. This won't kill the victim, but will cause the ant's (technically dead) body to wander aimlessly for days, until the ant's head simply drops off from its body. The maggot will the use the severed head as a pupation chamber, transforming into a new fly and finding itself a mate.

Glyptapanteles
There are many species of parasitoid wasp whose larvae develop in the bodies of other insects, particularly caterpillars, and there are many of these which can alter their host's behavior, but Glyptapanteles may be one of the most shocking. Like other parasitoid wasps, the larvae will eventually eat their way out of their caterpillar host to spin cocoons and develop into adults, but in this case, the process does not kill the caterpillar. Instead, the partially eaten host will stand guard over the wasp cocoons, cover them in layers of silk and flail viciously at trespassing insects. When the parasites are finished their metamorphisis and emerge from their cocoons as wasps, the zombie caterpillar finally dies of starvation and exhaustion.

Fish Flukes
For most fish, evading predatory birds is as simple as swimming just beyond the reach of a beak, so just how do so many fish end up on the bellies of pelicans and cranes? A huge portion of the average seabird's diet consists not of normal, healthy fish, but fish under the influence of parasitic worms. Sticklebacks, for example, suffer from the tapeworm Schistocephalus solidus, which grows so large that the host becomes swollen and sluggish. It also changes the host's coloration to be easier to spot, and finally, alters the host's behavior to swim near the surface. The worms feed the fish to the birds, and the birds spread the worms to new lakes and rivers in their droppings.