Holly ...

From Marginal Revolution

Toxoplasma gondii is a favorite parasite of evolutionary biologists because it has an incredible property. The parasite lives in the guts of cats where it sheds eggs in cat feces that are often eaten by rats. Now how to get back from the rat to the cat? Amazingly, Toxoplasma gondii infects the brains of rats making them change their behavior in a subtle way that increases the genetic fitness of the parasite. Toxoplasma makes the infected rats less scared of cats and so more likely to be eaten!