Toxoplasma gondii is a unicellular protozoan. The definitive hosts, cats, produce hardy oocysts and sporozoites. Ingestion by a nonfeline leads to the formation of tachyzoites acutely, which cause parasitemia and further dissemination, and bradyzoites, which lead to latent infection with the formation of tissue cysts in skeletal muscle, heart muscle, and central nervous system (CNS) tissue. Toxoplasmosis can be transmitted to humans by ingestion of tissue cysts in raw or inadequately cooked infected meat or in uncooked foods that have come in contact with contaminated meat, by inadvertent ingestion of oocysts and sporozoites in cat feces, or transplacentally. Immunocompetent adults and adolescents with primary infection are generally asymptomatic, but symptoms may include mild malaise, lethargy, and lymphadenopathy. Specific treatment for nonpregnant adults and adolescents is not required. Immunosuppressed patients may experience more severe manifestations, including splenomegaly, chorioretinitis, pneumonitis, encephalitis, and multisystem organ failure. These patients are also prone to reactivation of latent infection involving the CNS. All patients with human immunodeficiency virus infection and CD4 counts <100 cells per cubic millimeter should be treated prophylactically with pyrimethamine-sulfonamide. Congenital toxoplasmosis is marked by the classic triad of chorioretinits, intracranial calcifications, and hydrocephalus. Current studies have determined that prolonged treatment (1-2 years) of neonates with fansidar is important to prevent serious sequelae. Diagnosis of acute toxoplasmosis is mainly by antibody detection and generally only undertaken in pregnant patients with risk factors for transplacental transmission. All positive screening tests in pregnant women must be confirmed at a toxoplasma reference laboratory. Recent studies have shown that polymerase chain reaction testing of amniotic fluid is useful for identification or exclusion of fetal T. gondii infection. Ultrasound can be used as an adjunct to serological screening but cannot itself definitively diagnose disease. Early-first-trimester maternal infections are less likely to result in congenital infection, but the sequelae are more severe. Transplacental passage is more common when maternal infection occurs in the latter half of pregnancy, but fetal injury is usually much less severe. Typically, infected pregnant patients are treated with pyrimethamine-sulfonamide for positive PCR-amniotic-fluid testing and with spiramycin for negative PCR-AF testing.