In north America and Europe, the opportunistic infections from which patients with acquired immune deficiency syndrome (AIDS) frequently suffer are Pneumocystis carinii pneumonia and Mycobacterium avium-intracellulare: in central Africa these infections are uncommon or non-existent. Serious infections with Entamoeba histolytica and Strongyloides stercoralis would be expected to occur in AIDS patients: they do not. Falciparum malaria might be expected to interact with HIV infection: it does not. The epidemiology and pathophysiology of these infections are discussed with respect to HIV co-infection.