Pulmonary tuberculosis is an infectious disease caused by slow- growing bacteria that resembles a fungus, Myobacterium tuberculosis, which is usually spread from person to person by droplet nuclei through the air. The lung is the usual infection site but the disease can occur elsewhere in the body. Typically, the bacteria from lesion (tubercle) in the alveoli. The lesion may heal, leaving scar tissue; may continue as an active granuloma, heal, then reactivate or may progress to necrosis, liquefaction, sloughing, and cavitation of lung tissue. The initial lesion may disseminate bacteria directly to adjacent tissue, through the blood stream, the lymphatic system, or the bronchi.

Most people who become infected do not develop clinical illness because the body’s immune system brings the infection under control. However, the incidence of tuberculosis (especially drug resistant varieties) is rising. Alcoholics, the homeless and patients infected with the human immunodeficiency virus (HIV) are especially at risk. Complications of tuberculosis include pneumonia, pleural effusion, and extrapulmonary disease.

TB results from infection by any of the TB complex mycobacteria, including Mycobacterium tuberculosis, M bovis, M africanum, M microti, and M canetti.5

TB can be divided into primary, progressive-primary, and postprimary forms on the basis of the natural history of the disease. Postprimary TB results from either reactivation of a latent primary infection or, less commonly, from the repeat infection of a previously sensitized host. The term “postprimary” is preferred to “reactivation” when referring to the clinical diagnosis because firmly distinguishing recurrence from an antecedent infection is impossible in most cases. Approximately 10% of all infected patients are likely to develop reactivation, and the risk is highest within the first 2 years or during periods of immunosuppression.

The major determinants of the type and extent of TB disease are the...