Millions of people — fully one-third of the world’s population — are infected with the TB bacterium. Five to 10 percent of those will develop the active disease, for reasons we only partially understand. The bacteria can lie dormant for many years, and become active when the host is weakened by factors such as stress, poor nutrition, diabetes, or HIV infection. Once active, TB destroys tissue in the lungs and becomes contagious. Because it is airborne, there is little that can be done to protect against infection.
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