Cats are the only animal known to transmit toxoplasmosis to humans. Virtually all cats acquire toxoplasmosis, most often as young cats. The greatest risk to humans is during the three weeks after the cat acquires toxoplasmosis, when litter boxes are used, and when litter boxes are not cleaned daily. Toxoplasmosis in cat feces can become airborne after twenty-four hours. Toxoplasmosis can be transmitted from mother-to-fetus, with devastating consequences to the fetus; thus, pregnant mothers are warned to avoid litter boxes.

Medical doctors are taught acute toxoplasmosis is a mild and self-limiting disease, of little concern unless the patient is pregnant, HIV positive, or less than five years old. Acute toxoplasmosis can become chronic toxoplasmosis, and can cause a variety of chronic diseases and mental illness. Toxoplasmosis can cause depression, anxiety, obsessive compulsive disorder, bipolar disorder, schizophrenia, hostility, and anger. Toxoplasmosis increases testosterone, and has been linked to outbursts of anger and aggression, lowered inhibition, reckless and dangerous behavior, an increased risk of car accidents, self-harm, and self-mutilation. Scientists found toxoplasmosis played a role in intermittent explosive disorder, which is characterized by recurrent, impulsive, problematic outbursts of verbal or physical aggression that are disproportionate to the situations that trigger the outbursts.