Mycobacterium avium complex, or MAC, is a type of bacterial infection that can cause life-threatening symptoms in people who have compromised immune systems. People who have healthy immune systems may also be infected with MAC. However, the symptoms they experience—usually involving the lungs—are not usually life threatening. In people with advanced HIV disease, MAC usually doesn't involve the lungs. Instead, it causes disease in other organs, including the liver, the spleen, and the bone marrow and is then considered a "disseminated" infection.

MAC organisms can be found virtually anywhere in the environment. They live in water, soil, foods, and a variety of animals. As a result, it is difficult to avoid coming into contact with MAC. However, it is possible to prevent MAC from causing disease and symptoms in HIV-positive people using drugs (prophylaxis), a type of prevention that is almost always recommended for HIV-positive people with compromised immune systems (less than 50 CD4 cells).

MAC is preventable. One of the best ways to prevent MAC is to avoid having CD4 cell counts drop below 100 by starting antiretroviral HIV therapy. In people whose CD4 counts do not respond adequately to antiretroviral therapy, there are prophylactic drugs that can be taken to prevent MAC. Without antiretroviral drugs or prophylactic medication the incidence of MAC in people with HIV and low CD4 counts can be as high as 40 percent.