There are parasites that infect certain insects and turn them into self-destructive zombies (and it turns out this can happen to people too--do you own a cat?)
When a shrimp-like creature called a gammarid gets infected by a certain type of worm, it becomes a zombie and swims towards the water's surface, where they can easily be picked off by birds, which the worms need to complete their life cycle.
A spider that lives in the rain forests of Costa Rica abandons its web when it's compelled to build a nest that it shares the wasp that lives inside it, which then crawls out of the spider's eggs when she lays them. In order to manipulate the spiders, the parasite produces proteins that alter spider's behavior.
A virus that is commonly found on cabbage leaves is not harmful to humans, but it DOES infect certain species of caterpillars, signaling their brains in order to compel them to climb up to the tops of trees, where (again) they can easily be eaten by birds.
In the December 11th edition of the New York Times, Carl Zimmer quotes entomologist David P. Hughes as saying, "The infected individuals are out there, just eating and eating. They’re stuck in a loop."
When toxoplasma (a parasite that also infects humans) invades male rats, it causes them to produce extra testosterone, making them more attractive to female rats. When they mate, the spread the parasite to the females.
Testosterone also makes creatures less fearful, meaning that infected rats are less likely to be wary of a nearby cat. When the cat catches them, it becomes infected with the parasite as well.
Zimmer quotes entomologist Shelley Adamo as saying, "The knowledge that parasites can manipulate their hosts is old. The new part is how they do it."
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