Toxoplasma gondii is a single-celled organism which causes Toxoplasmosis, the most common parasitic infection worldwide affecting animals as well as humans. Cats are the only animal in which sexual reproduction of the organism occurs, and for this reason cats are the only domestic animal which has the potential to shed T. gondii eggs. When disease does occur, it may develop following primary infection due to an inadequate immune response to stop the invasive tachyzoites or as a result of a reactivated infection due to a compromised immune system. The most common symptoms of the disease are anorexia, weight loss, lethargy, dyspnea, ocular symptoms and pyrexia. As toxoplasmosis can be transmitted to humans it represents a serious health risk for people living in close contact with infected animals. Infection is especially dangerous for people with suppressed immune systems and cancer patients as well as pregnant women.