NTMs are closely related to tuberculosis bacteria and can (but do not need to) cause infectious diseases in humans. They are called "nontuberculous mycobacteria" (NTMs) to distinguish them from tuberculosis bacteria. There are approximately 200 different species and subspecies of these bacteria. Patients with bronchiectasis (dilation of the airways) are particularly affected. It is not uncommon for a chronic infection to develop. The most common pathogens are Mycobacterium avium complex, Mycobacterium kansasii, Mycobacterium xenopi and Mycobacterium abscessus. Unlike tuberculosis, there is practically no known risk of infection with nontuberculous mycobacteria.