What happens when someone gets infected with M. tuberculosis?
Between two to eight weeks after being infected with M. tuberculosis, a person’s immune system responds to the TB germ by walling off infected cells. From then on the body maintains a standoff with the infection, sometimes for years. Most people undergo complete healing of their initial infection, and the bacteria eventually die off. A positive TB skin test, and old scars on a chest x-ray, may provide the only evidence of the infection. If, however, the body’s resistance is low because of aging, infections such as HIV, malnutrition, or other reasons, the bacteria may break out of hiding and cause active TB. How is TB diagnosed? Doctors can identify most people infected with M. tuberculosis with a skin test. They will inject a substance under the skin of the forearm. If a red welt forms around the injection site within 72 hours, the person may have been infected. This doesn’t necessarily mean he or she has active disease. Most people with previous exposure to M. tuberculosis will test p
