Wow. Mood-altering cat parasites make women friendly and men into jerks:
A parasite that causes rats to sacrifice themselves to cats may also change human behavior, making women more outgoing and warmhearted, and men more jealous and suspicious. The Toxoplasma bacteria is shed in cat feces, which are eaten by rats; infected rats become fearless in the presence of cats, which makes them easier to catch, which, in turn spreads the disease to new cats.
Carl Zimmer has a few more examples that ended up in his Parasite Rex:
The lancet fluke Dicrocoelium dendriticum, for example, forces its ant host to clamp itself to the tip of grass blades, where a grazing mammal might eat it. It’s in the fluke’s interest to get eaten, because only by getting into the gut of a sheep or some other grazer can it complete its life cycle. Another fluke, Euhaplorchis californiensis, causes infected fish to shimmy and jump, greatly increasing the chance that wading birds will grab them.