Protozoan infections, against which immunity is predominantly T cell mediated, are likely to be more severe in patients with the acquired immune deficiency syndrome (AIDS) than in immunocompetent hosts. Leishmaniasis, toxoplasmosis and cryptosporidiosis are examples, the last two being particularly common in AIDS patients. Cerebral toxoplasmosis almost always results from recrudescence of latent infections acquired earlier in life. Depletion of T-helper (CD4+) lymphocytes enables bradyzoites to survive if released from cysts in the brain of patients. In the absence of immune pressure bradyzoites revert to tachyzoites and multiply to cause a rapidly developing, necrotizing encephalitis which needs immediate treatment. AIDS patients, especially those who are negative for antibodies to Toxoplasma, should avoid cats, the source of oocysts, and undercooked meat which may contain tissue cysts, as primary infections may become systemic. Cryptosporidium infections are more likely to be primary infections. Sources of infection are other people, farm animals and pets and there is a significant risk from contaminated domestic water supplies. As infections cause a life-threatening secretory diarrhoea in AIDS patients, for which there is not satisfactory treatment at present, such patients should take steps to minimize the risk of infection.