Toxoplasma gondii is a protozoan that can infect nearly any warm blooded mammal, including humans. But it can only sexually reproduce in cats' bellies. It's harmless to them...less so to others. When T. gondii winds up in other mammals (it travels around via cat poop), it replicates asexually and embeds itself into organs—even the brain—as little cysts. Still, it always wants to get back to a cat. So in animals like mice and rats—cat lunch and dinner, respectively—the protozoan fiddles with its host's brain, altering its behavior in ways that make it fearless and even attracted to felines. This, of course, leads to a dead rat and a happy T. gondii back home inside a kitty. Because you were wondering: T. Gondii is prevalent in humans, but is kept in check by a healthy immune system. Pregnant women and people with weak immune systems, however, can develop toxoplasmosis, a serious and sometimes fatal illness. Some studies have suggested that the parasite could also affect behavior and personality in people, but this is still pretty contested.

David Merrigan via Flickr