The infection can develop after inhaling droplets sprayed into the air as 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 infection is usually has no symptoms. In the United States, 95% of individuals will have healing of their primary tuberculous lesions with no further evidence of disease. 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, because of their weaker immune systems.
In disseminated disease, organs and tissues affected can include: the lining of the heart (pericardium), lining of the abdominal cavity (peritoneum), larynx, bronchus, cervical lymph nodes, bones and joints, organs of the male or female urinary and reproductive (genitourinary) system, eye, stomach, lining of the brain and spinal cord (meninges), and skin.
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.
An increased incidence of TB has been seen recently in the United States. Factors that may be causing this increase is tuberculous infection in people with AIDS and HIV infection, and increasing number of homeless people. Another factor is the development of drug-resistant strains of TB.
Incomplete treatment of TB infections (such as not taking medications for the prescribed length of time) can contribute to the proliferation of drug-resistant strains of bacteria. Disseminated TB is more common in AIDS patients.
About half of AIDS patients with a CD4 count less than 200 who develop TB will have disseminated disease (not localized disease, as in pulmonary tuberculosis).
