Mycobacterium avium complex: A serious opportunistic infection that is caused by two similar bacteria, Mycobacterium avium and Mycobacterium intercellulare, which are found in the soil and in dust particles. Abbreviated MAC. In persons with suppressed immune systems, such as people with AIDS, MAC can spread through the bloodstream to infect lymph nodes, bone marrow, the liver, the spleen, spinal fluid, the lungs, and the intestinal tract. Typical symptoms of MAC include night sweats, weight loss, fever, fatigue, diarrhea, and enlarged spleen. Antibiotics are commonly used in MAC prevention (for persons with suppressed immune systems) and treatment.