Fever can, among other things, be due to tuberculosis or opportunistic infections. Investigations in an AIDS patient with fever should be aimed at detecting treatable causes. Malaria is not an opportunistic infection, but can also occur in HIV patients. Recurrent Salmonella septicaemia is frequent. The reason is that Salmonella bacteria are facultative intracellular pathogens. They are normally eradicated by T-cell-activated macrophages. This mechanism is deficient in AIDS patients. A “functional hypogammaglobulinaemia” exists despite the polyclonal B-cell stimulation and the accompanying hypergammaglobulinaemia. There is an increased risk of infections with encapsulated bacteria (e.g. pneumococci), but also with Branhamella, Haemophilus and Staphylococcus. Infections with Mycoplasma and Legionella are not more frequent in seropositive persons. “Drug fever” occurs more frequently in seropositive than in seronegative persons.