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Pneumonia is caused by a variety of organisms, therefore depending on the organism that causes a particular case of pneumonia, the time that one is contagious varies. Some cases of pneumonia are spread by aerosolized particles (airborne infectious organisms), but most are acquired by organisms that are already in your mouth. Tuberculosis can also cause pneumonia, and it can take two weeks or longer of therapy before an infected person is no longer contagious. The most common types of pneumonia, like pneumococcal pneumonia, require other conditions besides exposure to an infected person to spread. For more, please read the Pneumonia article.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
"Epidemiology, pathogenesis, and microbiology of community-acquired pneumonia in adults"