Toxoplasma gondii is a parasite that can affect any mammal, but is generally found in cats. Humans can become infected by eating raw meat or uncooked vegetables or by handling cat feces. Usually, there are no symptoms or ill side-effects, unless the immune system is weakened. In that case numerous types of toxoplasmosis can develop. This can lead to muscle aches and pains, hepatitis, pneumonia, eye damage, and brain damage. Pregnancies can be aborted or still-born due to the disease, and successful births can leave the child with symptoms that can be obvious at birth or develop later in life.
Source: Ohio State University