When HIV enters the body it destroys white blood cells (CD4) which form part of the body's immunity. This means that people with HIV/AIDS have low immunity so they can catch infections more easily than people without HIV/AIDS. These illnesses are called 'opportunistic infections'. This does not mean that everyone with a disease has HIV. TB is the most common disease in Africa that affects people with HIV but not all people with TB have HIV.
Attribution: Straight Talk Foundation