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Spread of tuberculosis consists of three phases: In the transmission phase, Mycobacterium tuberculosis is transferred from a source of a host via aerosolization of respiratory secretions. During the infective phase, the organism establishes itself within the lung of the new host. In the pathogenetic phase, organism- and host-related mechanisms bring about clinical disease. In the past, the vast majority of cases of tuberculosis in the United States were assumed to be reactivation of dormant disease. However, the pathogenesis has increasingly shifted to progressive primary disease. This shift may be due to the low endogenous resistance to tuberculosis in native-born Americans. In addition, various social problems, especially in large cities, favor person-to-person transmission. Infectivity of M tuberculosis itself may be increasing as strains resistant to multiple agents become more common. |
