Tuberculosis (TB) is caused by organisms of the Mycobacterium tuberculosis complex, which includes M. tuberculosis, the most common and important agent of human mycobacterial disease, and M. bovis, which (like several other mycobacterial species) is acquired via ingestion of unpasteurized milk. M. tuberculosis is a thin aerobic bacillus that is neutral on Gram's staining but that, once stained, is acid-fast; i.e., it cannot be decolorized by acid alcohol because of the cell wall's high content of mycolic acids and other lipids.