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:

• increase in HIV infection
• increasing number of homeless individuals (poor environment and poor nutrition)
• the appearance of drug-resistant strains of TB

Incomplete treatment of TB infections (such as failure to take medications for the prescribed length of time) can contribute to the emergence of drug-resistant strains of bacteria. Individuals with damaged immune systems from AIDS have a higher risk of developing active tuberculosis- either from new exposure to TB or reactivation of dormant mycobacteria. In addition, without the aid of an active immune system treatment is more difficult and the disease more resistant to therapy.

The incidence of tuberculosis in the U.S. has been around 10 per 100,000 people but varies dramatically by area of residence and socio-economic class. Also see:

• disseminated tuberculosis (affects the whole body)
• atypical mycobacterial infection

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