Toxoplasma gondii is a parasite that infects mice and makes them less afraid of cats. The fearless mice are more likely to be eaten by cats, and the parasite reproduces in the cats' digestive tract. Toxoplasma's ability to alter mouse behavior has led to a bit of evidence and a lot of theories about how human brains might be affected by the parasite. Since about a third of adult humans worldwide have toxoplasmosis, this is a pretty important question.
The most obvious starting point is behavior driven by fear or fearlessness. According to Stanford researcher Robert Sapolsky, "Two different groups independently have reported that people who are Toxo-infected have three to four times the likelihood of being killed in car accidents involving reckless speeding." Patrick House of Slate observes that countries where rates of toxoplasmosis are higher have better soccer teams. Less obvious but perhaps more intriguing is a possible link between toxoplasmosis and schizophrenia. According to Physorg:
Evidence that T. gondii infections may be a cause of schizophrenia, while not yet conclusive, is growing, [Johns Hopkins researcher] Yolken said. A review of past studies, published last year by Yolken and Torrey, collected a variety of intriguing correlations. For example: People with schizophrenia have a higher prevalence of T. gondii antibodies in their blood. There are unusually low rates of schizophrenia and toxoplasmosis in countries where cats are rare, and unusually high rates in places where eating uncooked meat is customary. And some adults with toxoplasmosis show psychotic symptoms similar to schizophrenia.
Studies have linked a history of toxoplasmosis with increased rates of other mental changes, too, including bipolar disorders and depression. A 2002 study in the Czech Republic noted slowed reflexes in Toxoplasma-positive people and found links between the infection and increased rates of auto accidents.
A University of Maryland study last year found that people with mood disorders who attempt suicide had higher levels of T. gondii antibodies than those who don't try to take their own lives. Still, the links between schizophrenia and toxoplasmosis are not simple. For example, most people infected with T. gondii never become schizophrenic. And not all schizophrenics have been exposed to Toxoplasma.
Even seemingly clear-cut research results are open to interpretation: witness Sapolsky talking about recklessness while the Physorg article connects the auto accidents to slow reflexes. The practical question of whether the symptoms of schizophrenia can be effectively and efficiently reduced by treating toxoplasmosis--which is itself a difficult task due to T. gondii's talent for hiding and protecting itself--remains to be answered. Nonetheless, it's always interesting to see physical causes suggested for mental illness, even as we stay wary of anyone offering a simple cure.