evidence: Up to 50 percent of global population is infected by the 'cat parasite' Toxoplasma gondii, and in some areas, the infection rate is as high as 95 percent. Unless you get tested, there's no way of knowing that you're infected, but it's been associated with a number of mental disorders, such as schizophrenia and bipolar disorder. [...] For the uninitiated, Toxoplasma is a cat-borne parasite that causes Toxoplasmosis - a disease that is considered 'asymptomatic' in most healthy people, but can lead to a range of other disorders in those with compromised immune systems, such as pregnant women and the elderly.