Toxoplasma gondii, a small parasite less than the size of a human cell, causes the disease toxoplasmosis. T. gondii infects approximately one-third to one-half of all people worldwide. People can acquire the parasite when they eat meat that is not cooked to well done. They can also acquire the parasite directly or indirectly from a domestic cat (or another member of the cat family, such as a bobcat). An acutely infected cat excretes millions of T gondii in a dormant form called an oocyst. Even one of these oocysts can cause infection, and can remain infectious in water for up to six months or in warm, moist soil for up to a year. An oocyst may remain on anything that touches it. For example, it may be transferred from a cat to garden soil, to a vegetable in the garden, and finally to someone who eats the vegetable. It is not uncommon for someone to become infected by T. gondii without knowing the source of the infection
If a mother is infected for the first time while she is pregnant and passes the parasite to her child, the baby can suffer eye or brain damage. Babies infected with T. gondii while in the womb are said to have congenital toxoplasmosis. Older children and adults who become infected may not have symptoms, or they may develop a flu-like illness or enlarged lymph glands. In rare cases, older children and adults who are infected develop other conditions such as brain or heart inflammation.
T. gondii can live in the body in dormant form and can cause toxoplasmosis if a person becomes immune compromised from conditions such as cancer, autoimmune diseases, AIDS, or transplantation and associated treatments. It also can resurface and cause eye disease in congenitally infected children later in life and in some older children and adults who were infected after birth.
Treatment can prevent transmission of the parasite from mother to child. When diagnosed and treated early, treatment can also prevent the infection's adverse effects for a baby or for older persons with brain or eye disease. Medicines currently used to treat toxoplasmosis have side effects. No medicines currently available can eliminate the dormant parasites from the body, so toxoplasmosis may recur. However, there is promise for both improved medicines and a vaccine. A chronically infected mother's immune response (if she is not immune compromised) prevents her from transmitting the parasite to her baby. Researchers are encouraged by this response, and are seeking a vaccine that will elicit a similar protective immune response to prevent infection. There is also encouraging progress being made towards discovery of improved medicines.