Most of the time they cause no harm, but they can cause infection in groups with certain risk factors. MAC organisms are common in soil and water and are easily inhaled during daily activities. MAC is one of a large group of nontuberculous mycobacteria (NTM), and the most common cause of NTM lung disease in the U.S. MAC includes two closely related species, Mycobacterium avium and Mycobacterium intracellulare, and may also be referred to as MAI.
MAC lung disease is an infection caused a group of bacteria called Mycobacterium avium complex (MAC).