Diseases caused by mycobacteria, a diffuse group of acid-fast, rod-shaped bacteria in the genus Mycobacterium. Some mycobacteria are saprophytes, while others can cause disease in humans. The two most important species are M. tuberculosis (the cause of tuberculosis) and M. leprae (the cause of leprosy); other species have been called by several names, particularly the atypical mycobacteria or the nontuberculous mycobacteria. This article deals mainly with nontuberculous mycobacteria.