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Mycobacterium (a genus of Actinobacteria in family Mycobacteriaceae) includes pathogens known to cause serious diseases in mammals, including tuberculosis (M. tuberculosis) and leprosy (M. leprae). Lesser known disorders include mycobacterium avium complex (MAC) and hypersensitivity pneumonitis among others. Mycobacterium are aerobic non-motile Gram-positive, classified as an acid-fast Gram positive straight or slightly curved rods. Many species adapt readily to growth on very simple substrates while other species can be very difficult to culture (fastidious). A natural division occurs between slowly and rapidly growing species; Mycobacteria that form colonies clearly visible to the naked eye within seven days on subculture are termed rapid growers, while those requiring longer periods are slow growers. Mycobacteria are widespread organisms, typically living in water and food sources. Some, however, including the tuberculosis and the leprosy organisms, are obligate parasites and are not found as free-living members of the genus. Mycobacterial infections are notoriously difficult to treat.