Toxoplasmosis is a caused by a parasite, which lives in the host as long as the host lives. Cats are the only known animal reservoir for toxoplasmosis. Most cats are infected early in life, and are most infectious in the weeks following acute infection. Transmission to humans is by ingestion or inhalation of microscopic particles of infected feces; and toxoplasmosis can cross the placenta to infect a fetus. Toxoplasmosis has been ignored in chronic disease, and believed to be a self-limiting, largely asymptomatic disease; which is only of concern when the patient is under five, pregnant or immunosuppressed.

Toxoplasmosis can cause a variety of mental illnesses, including depression, anxiety, obsessive compulsive disorder, bipolar disorder, schizophrenia, hostility, anger, emotional outbursts disproportionate to the circumstance, and reckless behavior. Toxoplasmosis has been “associated” with reckless and dangerous behavior, including an increased risk of car accidents, self-harm, and self-mutilation. Toxoplasmosis increases testosterone in male brains, and has been linked to outbursts of anger and aggression. Patrick House, a Stanford neuroscientist, reports males with toxoplasmosis are more aggressive and less inhibited. University of Chicago researchers found toxoplasmosis played a role in impulsive aggression; and found a link between toxoplasmosis and intermittent explosive disorder (IED), a disorder characterized by recurrent, impulsive, problematic outbursts of verbal or physical aggression that are disproportionate to the situations that trigger the outbursts.

Mental illness can be caused by a surprising variety medical illnesses and infectious pathogens.