Tuberculosis is caused by infection with Mycobacterium tuberculosis (M. tuberculosis).11 Infection occurs through inhalation of airborne droplets containing viable M. tuberculosis, which then travel to the alveoli of the lungs. Only people with active disease can spread the infection.
M. tuberculosis is transmitted through infectious airborne particles, known as “droplet nuclei,” which can be generated when people with pulmonary or laryngeal tuberculosis sneeze, cough, speak or sing.11, 13 These small particles (1 to 5 micrometers in diameter) can stay suspended in the air for hours.14 Non-coughing individuals who are suspected of having tuberculosis cannot be presumed to be non-infectious because M. tuberculosis transmission may still occur without the presence of coughing.15 According to one study, up to 77% of respiratory bio-aerosol samples from newly diagnosed patients may contain M. tuberculosis organisms.16
If a susceptible person inhales aerosolized droplet nuclei containing M. tuberculosis, infection may begin if the organisms reach the alveoli. Within two to 12 weeks, the body’s immunological response to M. tuberculosis usually prevents further multiplication and spread.11 The mycobacterium can live in the lungs of an infected person for years, even a lifetime, without the person exhibiting any symptoms; this state is called latent infection.11 A person with latent tuberculosis is generally asymptomatic and not infectious to others but the infection can develop into active tuberculosis in the future and usually exhibits a positive reactive tuberculin skin test.11
Most people who have latent tuberculosis infection never develop active disease, but if they do not receive treatment for latent infection, about 10 percent of people with latent infections can develop active disease over a lifetime.17 This can happen when the person’s immune system is weakened, allowing the mycobacteria to cause active tuberculosis infection (e.g., individuals with HIV, diabetes, certain hematologic disorders such as leukemias and lymphomas, prolonged corticosteroid use, and other conditions).18
Only a person with active tuberculosis can transmit the disease. People with active tuberculosis infection generally have symptoms (e.g., persistent, productive cough; night sweats, fever, weakness or fatigue; weight loss; pain in the chest); and can have a positive tuberculin skin test reaction.11