Tuberculosis is a contagious bacterial infection caused by Mycobacterium tuberculosis (TB). The lungs are primarily involved, but the infection can spread to other organs.

Tuberculosis can develop after inhaling droplets sprayed into the air from a cough or sneeze by someone infected with Mycobacterium tuberculosis. The disease is characterized by the development of granulomas (granular tumors) in the infected tissues. The usual site of the disease is the lungs, but other organs may be involved. The primary stage of the infection is usually asymptomatic. In the United States, the majority of people will recover from primary TB infection without further evidence of the disease. Pulmonary TB develops in the minority of people whose immune systems do not successfully contain the primary infection. The disease may occur within weeks after the primary infection, or it may lie dormant for years before causing disease. Infants, the elderly, and individuals who are immunocompromised (for example, those with AIDS, those undergoing chemotherapy, or transplant recipients taking antirejection medications) are at higher risk for progression to disease or reactivation of dormant disease. In pulmonary TB, the extent of the disease can vary from minimal to massive involvement, but without effective therapy, the disease becomes progressive.

The risk of contracting TB increases with the frequency of contact with people who have the disease, and with crowded or unsanitary living conditions and poor nutrition. An increased incidence of TB has been seen recently in the United States. Factors that may contribute to the increase in tuberculous infection are: