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U.S. Department of Health & Human Services

The process of becoming infected with TB begins when inhaled TB bacteria, also known as tubercle bacilli, begin to multiply in the small air sacs of the lungs.

Some TB bacteria then enter the bloodstream and spread throughout the body. Usually within 2 to 8 weeks, the immune system intervenes, halting multiplication and preventing further spread.

At this point, the person has latent TB infection, because the immune system has halted the process, keeping the TB bacteria under control. People with latent TB infection do not feel sick, and they cannot spread TB to others.

Close up illustration of rod-shaped TB bacteria (in red) in the alveoli of the lung, surrounded by special immune cells that form a barrier shell around the bacilli. The barrier shells keeps the TB bacteria contained and under control.