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.: Transmission :. Tubercle bacilli are spread by airborne dissemination of droplet nuclei which are inhaled and deposited in the alveoli. If a person with sputum positive for AFB exhales or coughs, an aerosol containing tubercle bacilli is disseminated. One cough can produce 3,000 droplet nuclei (i.e. infectious droplets). The individual's risk of exposure depends on the concentration of droplet nuclei in the contaminated air and the length of time he breathes that air. The risk of infection of a susceptible individual is high with close, prolonged, indoor exposure to a person with sputum smear positive TB. It has been estimated that one undiagnosed smear-positive case may infect about 10 people in one year. The risk of transmission of infection from a person with sputum smear negative TB is low and with extra-pulmonary TB is even lower. Tubercle bacilli can remain airborne starting from minutes to hours after expectoration of infectious agent.
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Pathogenesis