Mycobacterium tuberculosis is the causative agent of tuberculosis (TB), which usually infects the lungs and infects any part of the body, such as the kidney, spine, and brain. As a respiratory disease, it is transmitted through infected respiratory droplets when someone coughs or sneezes. Not everyone infected with M. tuberculosis will develop TB, and only 10% of those who become infected will progress to TB disease. TB latent infection develops into TB disease when the bacteria overcome the body's immune system defenses, which can happen with aging, serious illness, stressful events, HIV infection, or other conditions. If not treated properly, TB can be fatal. Diagnosis is by chest X-ray, a skin test, or a serologic assay that detects the presence of TB-specific IgG or IgM antibodies.