Atypical mycobacteria are a group of bacteria that are widely distributed in nature.
They can be found in water, soil, animals and man, usually without evidence of disease. The atypical mycobacteria belong to the same family of mycobacterial organisms as M.tb but include other species such as M.avium, M. intracellularae, M. kansasii, M. xenopi, and M. fortuitum. Most infections with these organisms are believed to arise from environmental exposure to organisms in infected water, soil, dust, or aerosols. Many people become infected with and harbor atypical mycobacteria in their respiratory secretions without any symptoms or evidence of disease. In some individuals, however, infection with these organisms may result in disease involving the lungs, skin, or lymph nodes.