Toxoplasma infection in most adult animals and humans is asymptomatic because of effective protective immunity; this involves antibody acting extracellularly, and T-cell factors acting intracellularly. Whenever immunity is not acquired in a timely fashion, tachyzoites continue to multiply, destroying an excessive number of cells, producing lesions in several organs, with pneumonia and encephalitis the prominent causes of illness and death. However, immunity is insufficient to destroy the slowly multiplying bradyzoites persisting in tissue cysts in many organs - a parasite adaptation to await ingestion of one host by another. Toxoplasma cysts produce lesions when they disintegrate, because of the delayed type of hypersensitivity accompanying infections. In the presence of immunity, the released bradyzoites are destroyed, but when protective immunity fails, the bradyzoites can develop again into actively multiplying tachyzoites parasitizing and destroying cells in expanding foci, usually in the brain. In this review J.K. Frenkel discusses the complex interplay of immunological and parasite factors participating in the various lesions associated with acute and chronic Toxoplasma infections.