A chronic , contagious bacterial infection caused by Mycobacterium tuberculosis, which has spread to other organs of the body by the blood or lymph system . The infection can develop after inhaling droplets sprayed into the air as from a cough or sneeze by someone infected with Mycobacterium tuberculosis. The usual site of the disease is the lungs, but other organs may be involved.
Primary infection is usually asymptomatic .
Disseminated disease develops in the minority whose immune systems do not successfully heal the primary infection. The disease may occur within weeks after the primary infection, or may lie dormant for years before causing illness. Infants and the elderly are at higher risk for rapid progression to disease. The risk of contracting TB increases with the frequency of contact with people who have the disease, in crowded or unsanitary living conditions, and with poor nutrition. Hispanics, Native Americans, and blacks are at higher risk for developing the disease.