Tuberculosis is primarily caused by Mycobacterium tuberculosis (also called the tubercle bacillus). Humans can also develop bovine tuberculosis, a less serious disease that is primarily caused by drinking milk from cattle infected with Mycobacterium bovis.³ Other mycobacteria can cause tuberculosis in various animals, but they rarely cause disease in humans.⁴ As discussed later in this chapter, bovine and other mycobacteria may produce positive reactions to the tests used to screen people for tuberculosis.

Although M. tuberculosis was first identified in 1882, its behavior remains poorly understood in some important respects. In particular, much remains to be learned about the mechanisms of latent tuberculosis infection and disease activation and reactivation.

M. tuberculosis typically spreads through the air when people who have infectious tuberculosis in their respiratory tract cough, sneeze, speak, sing, or otherwise expel tiny particles containing the bacteria. A fine mist (an aerosol) of infectious particles can also be created in other ways, for