Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. This single-celled parasite is capable of living in a wide range of birds and mammals, but only produces eggs in the lining of the intestines of cats. In humans it usually causes few or no symptoms. On average, it is estimated that 10% to 20% of the population in North America is infected – that infection rate is higher in other parts of the world.
Toxoplasmosis usually lies dormant, but occasionally it reactivates to cause disease. Usually this happens when some other disease weakens the immune system. Toxoplasmosis is considered an opportunistic infection, one that shouldn't harm healthy people but can be very serious if your immune defenses are down (e.g., people with AIDS or cancer, or who are taking medications that suppress the immune system). It also threatens the fetus if an expectant mother is infected during pregnancy.