Toxoplasma gondii / Toxoplasmosis

This is the coolest parasite ever. It spends its life cycle in cats, but can live in other animals. It spends time in other animals mostly in brain tissue and is pretty clever about it. It's mostly harmless (a healthy human or cat can easily keep it completely at bay,) although may be harmful to fetuses, so it's a concern for pregnant women.

It lives and reproduces in a cat's stomache, and, like most parasites that do, it spreads through feces. But, here's the problem: cats avoid feces. Some other animals (like rats and mice) do not and since fecal matter is good fertilizer, it could end up in humans via plants.

Here's the awesome part: rats and mice are instinctively afraid of the scent of felid urine. This helps them to avoid cats and cat territory. Toxoplasmosis selectively alters their brain chemistry to make them attracted to this scent. It doesn't change any of their other fears (if the rat dies from something else, so does the parasite.) So now that rats are attracted to cats, T. gondii is suddenly and happily back in a cat's stomache!

Sinister, eh?

T. gondii has also been shown to have effects on humans, basically raising estrogen production in women and testosterone production in men.