Saw this book on the Daily Show a few nights ago and it looked interesting: Survival of the Sickest by Sharon Moalem
The author mentioned a theory of schizophrenia as due to toxoplasma infected cats, who are themselves infected by rats carrying the disease. Apparently, when the rat is infected, the toxoplasma alters the behavior of the rat such that it doesn’t run away from a cat. So, the parasite ensures its survival.
Although I’m not a big fan of just-so evolutionary explanations in general, this book sounds like it might be a fun read.