"The Prokaryotes" Second Edition, p. 1243 states: Mycobacterium avium complex (MAC) is now used to include both Mycobacterium avium and Mycobacterium intracellulare, and Mycobacterium fortuitum complex (MFC) to include both Mycobacterium fortuitum and Mycobacterium chelonae. This agrees with Runyon's suggestion that each complex is made up of two or more species whose distinction is of no medical importance. With MAC species, the concept generally applies, but identification to species and subspecies level has become important in recent years as a way to follow the epidemiology of disease in special population groups and as a guide to appropriate therapy. The term Mycoplasma avium-intracellulare has no taxonomic status, so it and its acronym MAI should not be used.