Toxoplasmosis gondii is an obligate intracellular protozoan parasite. Infection is due to ingestion of undercooked meat containing cysts or sporulated oocysts, transplacental transmission of trophozoites, or acquired through blood transfusion/organ transplantation. The organism affects the brain, retina, lungs, skeletal muscle, and heart.

In AIDS patients, the infection inovlves the basal ganglia (75%) and/or is scattered throughout the brain parenchyma at the gray-white junction. Approximately 40% of infections are solitary. Lesions demonstrate ring enhancement and surrounding edema. There is improvement with medical therapy. The differential diagnosis includes CNS lymphoma, especially if there is a single lesion.

Intrauterine infection demonstrates increased risk of placental transmission late in pregnancy. Chorioretinitis and mental retardation are common sequelae. Microcephaly is not present. Periventricular and choroid calcifications are a common feature.