Mycobacterium avium complex (MAC) consists of two species— M avium and M intracellulare. Because these species are difficult to differentiate, they are also collectively referred to as Mycobacterium avium-intracellulare (MAI) .

MAC causes disseminated infection in immunocompromised hosts, but only a minority of immunocompetent hosts with MAC infection develop MAC lung disease. Patients with underlying lung disease or immunosuppression may develop progressive MAC lung disease. M avium is the isolate in more than 95% of patients with AIDS who develop MAC infections; M intracellulare is responsible for 40% of such infections in immunocompetent patients. MAC is the most common cause of infection by nontuberculous mycobacteria (NTM) in patients with AIDS.

MAC is ubiquitous in distribution. It has been isolated from freshwater and saltwater worldwide. The common environmental sources of MAC include aerosolized water, piped hot water systems (including household and hospital water supplies), bathrooms,[1] house dust, soil, birds, farm animals, and cigarette components such as tobacco, filters, and paper.