HomeM. tuberculosis is a bacterium that is communicable from one human to another mainly by the aerosol route and rarely by other means such as ingestion or percutaneous inoculation (eg, through laboratory or hospital accident), or via solid organ or hematopoietic stem cell transplantation.1 The reservoir for M. tuberculosis is humans. Bovine TB, which in the past was caused by ingestion of milk heavily infected by Mycobacterium bovis that then penetrated the mucosa of the oropharynx or the gastrointestinal tract, has been much reduced globally and almost completely eliminated in Canada as a result of the pasteurization of milk and tuberculin testing of cattle.
The droplets in aerosols have an extremely slow settling rate (0.5 mm per second or less), which permits their transport by air currents, duct systems or elevator shafts for significant distances from the source case. Large particles settle quickly and are either not inhaled by contacts or, if inhaled, are trapped in the mucus of the upper airway. Only the droplet nuclei in the size range 1 to 5 microns reach the terminal air spaces or alveoli; each is understood to contain only a few bacteria.2,3
The likelihood of a transmission event will depend on the number of infectious droplet nuclei per volume of air (infectious particle density) and the length of time that the uninfected individual spends breathing that air.
Switch To: Français