Opportunistic infections and diseases occur in people whose immune systems have been weakened. They usually do not occur in people who have healthy immune systems. When the immune system is weakened by a disease such as HIV infection, opportunistic diseases may cause serious, even life-threatening, illnesses.

Opportunistic infections that are common in people who have AIDS include:

• Pneumocystispneumonia.
• Tuberculosis.
• Toxoplasmosis.
• Cytomegalovirus (CMV).
• Mycobacterium avium complex (MAC).

People who have AIDS are more likely to develop certain cancers, such as:

• Kaposi's sarcoma.
• Cancer of the lymphatic system (lymphoma).
• Cancer of the cervix.