When people suffering from active pulmonary TB cough, sneeze, speak, kiss, or spit, they expel infectious aerosol droplets 0.5 to 5 µm in diameter. A single sneeze, for instance, can release up to 40,000 droplets.[17] Each one of these droplets may transmit the disease, since the infectious dose of tuberculosis is very low and the inhalation of just a single bacterium can cause a new infection.
People with prolonged, frequent, or intense contact are at particularly high risk of becoming infected, with an estimated 22% infection rate. A person with active but untreated tuberculosis can infect 10–15 other people per year.[2] Others at risk include people in areas where TB is common, people who inject illicit drugs (especially when sharing needles), residents and employees of high-risk congregate settings, medically under-served and low-income populations, high-risk racial or ethnic minority populations, children exposed to adults in high-risk categories, patients immunocompromised by conditions such as HIV/AIDS, people who take immunosuppressant drugs, and health care workers serving these high-risk clients.[19]
Transmission can only occur from people with active—not latent—TB. The probability of transmission from one person to another depends upon the number of infectious droplets expelled by a carrier, the effectiveness of ventilation, the duration of exposure, and the virulence of the M. tuberculosis strain.[10] The chain of transmission can therefore be broken by isolating patients with active disease and starting effective anti-tuberculous therapy. After two weeks of such treatment, people with non-resistant active TB generally cease to be contagious.[20]
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