Infectious diseases constitute the immediate cause of death in up to 90% of patients with advanced HIV. Some are caused by common pathogens, but many are opportunistic, meaning that they are caused by microbes which usually do not cause disease in the immunocompetent host. Knowledge of these is incomplete and new forms of opportunistic infections attributed to previously unrecognized and uncharacterized microbes are still being discovered.

The incidence and spectrum of these infections differ in important respects from those associated with other immunosuppressive disorders. Whereas all immunocompromised patients are vulnerable to Toxoplasma encephalitis, oral candidiasis and pulmonary tuberculosis, many opportunistic diseases including Pneumocystis carinii pneumonia (PCP), and systemic infections due to Cryptococcus neoformans, cytomegalovirus (CMV), the Mycobacterium avium complex and Cryptosporidium species have occurred more frequently in people infected with HIV.

The pattern of infection varies between different socio-ecological settings. In some African countries as many as 50% of patients with advanced HIV disease will develop tuberculosis. In contrast, Pneumocystis carinii pneumonia is less frequent in these countries. This may be because many patients die before their immune defences are severely attenuated or because of under-reporting. In the immunocompromised patient infections often present atypically; disseminated disease is common and two or more infections may occur concurrently. The systems commonly involved in manifestations of the opportunistic infections are given below

Other infections such as those due to M. tuberculosis, Shigella, and Salmonella species occur in people with normal immunity and are not classified as “opportunistic infections”. They are, however, included as they occur with increased frequency in people with HIV.