Toxoplasmosis is an infection caused by a single-celled parasite called Toxoplasma gondii. The parasite is commonly found throughout the world and tends to infect birds and mammals, including humans. The infection is usually acquired by accidentally ingesting something contaminated with the parasite, such as undercooked meat or contaminated soil. In most cases, toxoplasmosis does not cause any symptoms, and the immune system usually keeps the parasite from causing illness. However, pregnant women and people with weakened immune systems are at risk of severe consequences if infected. Symptoms of toxoplasmosis can include swollen lymph glands, muscle aches and pains, headache, fever, and inflammation of the lungs or heart muscle. In severe cases, the infection can cause damage to the brain, eyes, or other organs. Toxoplasmosis can also be passed from a pregnant person to the fetus, causing congenital toxoplasmosis, which can result in serious medical problems for the baby. Diagnosis of toxoplasmosis is usually based on a persons medical history, symptoms, and laboratory test results. Treatment is usually not necessary for otherwise healthy individuals who are not pregnant, but medication may be prescribed for those with severe symptoms or weakened immune systems.