Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most warm-blooded animals, including humans, but the primary host is the felid (cat) family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself recently been infected, or by transmission from mother to fetus. Cats have been shown as a major reservoir of this infection. While this is true, contact with infected undercooked meat seems to be a more important cause of human infection in many countries.

In Season one, episode 17, Dr Foreman and Dr House called toxoplasmosis a fungus. It is in fact a parasitic protista.

Up to one third of the world's population is estimated to carry a Toxoplasma infection. The Centers for Disease Control and Prevention notes that overall seroprevalence in the United States as determined with specimens collected by the third National Health and Nutritional Assessment Survey (NHANES III) between 1988 and 1994 was found to be 22.5%, with seroprevalence among women of childbearing age (15 to 44 years) of 15%. During the first few weeks, the infection typically causes a mild flu-like illness or no illness. After the first few weeks of infection have passed, the parasite rarely causes any symptoms in otherwise healthy adults. However, people with a weakened immune system, such as those infected with HIV, and fetuses, may become seriously ill, and it can occasionally be fatal. The parasite can cause encephalitis (inflammation of the brain) and neurological disorders and can affect the heart, liver, and eyes (chorioretinitis).

Recent research has shown that animals infected with toxoplasmosis are attracted to the scent of cat urine. This appears to be a behavioral change in the animal to allow it to be eaten by a cat, which re-infects the cat and allows the parasite to complete its life cycle. For example, normal rats avoid the scent of cats, but infected rats do not.

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