Opportunistic infections and diseases occur in people whose immune systems have been weakened by a disease such as HIV infection. Opportunistic infections that are common in people who have HIV include pneumocystis pneumonia, tuberculosis, salmonella, candidiasis, herpes, and toxoplasmosis. People who have HIV are also more likely to develop cancers such as Kaposi sarcoma and lymphoma.

If a person with HIV has one of these opportunistic infections or cancers, they may now be considered to have AIDS, the most serious stage of HIV infection.