The acquired immunodeficiency syndrome (AIDS) is the result of a human immunodeficiency virus (HIV) infection damaging the cell-mediated immune system. A wide range of opportunistic infections (OI) and tumours develop; additionally, HIV directly damages some organs. The patterns of opportunistic diseases (OD) are different in different parts of the world, depending on the local prevalence of latent and acquired infections and on the survival of HIV-infected patients. OD patterns change as people migrate. Recently introduced highly active anti-retroviral chemotherapy prevents many of the common OIs, but also introduces a new range of toxic pathological damage. Longer survival permits development of new HIV-related diseases. The pathology of HIV/AIDS is not static but changing.