Tuberculosis 1. Pathogenesis Infection with M. tuberculosis is usually acquired from inhalation of tiny airborne droplets (“droplet nuclei”) or free-floating bacilli. Droplet nuclei arise from the cough of an infected person. Because of their small size, the droplets remain suspended in air for hours. Rarely, TB is acquired from a draining wound, at autopsies, or by ingestion of contaminated milk. Infectivity is greatest in patients with cavitary disease. (Cavities contain huge numbers of bacteria, up to 10 9 bacilli. This huge burden of organisms is why combination therapy must be given for several months to prevent resistance.) A positive “acid-fast” (Ziehl-Neelsen) stain is a sensitive test for infectivity. Patients with negative sputum smears are not infectious and need not be isolated. (Occasional patients with early TB may have negative smears, but positive cultures.) Primary infection arises after droplet inhalation, usually into a lower lobe (because lower lobes are better ventilated than upper lobes). When an alveolar macrophage ingests a bacillus, it may be
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