OK, this one is a bit left-field and tenuous (so far), but a fascinating idea, especially to a Science Fiction nerd such as myself. Carl Zimmer posts on his blog about research into Toxoplasmosis, and the effect of drugs used to treat schizophrenia upon it. The overall premise is that Toxoplasma, a common parasite that completes its life cycle in cats and is probably present in a large section of the human population, has been observed to create abnormal behaviour patterns in rats, an intermediate host for the parasite. This behaviour manifests as a lack of fear of cats (which for a rat is tantamount to gaining a death wish). Some scientists have long believed that Toxoplasma may have a similar effect on humans, but this has always been a very controversial view. E. Fuller Torrey of the Stanley Medical Research Institute in Bethesda, Maryland had noticed similarities between schizophrenia and Toxoplasma, and performed an experiment that seems to show that the drugs that suppress schizophrenia in human patients also seem to retard the growth of Toxoplasma in human cell cultures. There’s a lot of work to be done to advance this theory further. But it could be that nearly half the human race is harbouring a parasite that alters the mental state and behaviour of its hosts.