1. A specific disease caused by infection with Mycobacterium tuberculosis, the tubercle bacillus, which can affect almost any tissue or organ of the body, the most common site of the disease being the lungs. Primary TB is typically a mild or asymptomatic local pulmonary infection. Regional lymph nodes may become involved, but in otherwise healthy people generalized disease does not immediately develop. A cell-mediated immune response arrests the spread of organisms and walls off the zone of infection. Infected tissues and lymph nodes may eventually calcify. The tuberculin skin test result becomes positive within a few weeks and remains positive throughout life. Organisms in a primary lesion remain viable and can become reactivated months or years later to initiate secondary TB. Progression to the secondary stage eventually occurs in 10–15% of people who have had primary TB; in one half of these, progression occurs within 2 years. The risk of reactivation is increased by diabetes mellitus, malnutrition, HIV infection, silicosis, and various systemic or malignant conditions, as well as in patients with alcoholism, IV drug abusers, nursing home residents, and those receiving adrenocortical steroid or immunosuppressive therapy. Secondary or reactivation TB usually results in a chronic, spreading lung infection, most often involving the upper lobes. Minute granulomas (tubercles), just visible to the naked eye, develop in involved lung tissue, each consisting of a zone of caseation necrosis surrounded by chronic inflammatory cells (epithelioid histiocytes and giant cells). These lesions, which give the disease its name, are also found in other tissues (lymph nodes, bowel, kidney, skin) to which the disease may spread. Rarely, reactivation results in widespread dissemination of tubercles throughout the body (miliary TB). The symptoms of active pulmonary TB are fatigue, anorexia, weight loss, low-grade fever, night sweats, chronic cough, and hemoptysis. Local symptoms depend on the parts affected. Active pulmonary TB is relentlessly chronic and, if untreated, leads to progressive destruction of lung tissue. Cavities form in the lungs, and erosion into pulmonary blood vessels can result in life-threatening hemorrhage. Gradual deterioration of nutritional status and general health culminates in death due to wasting, infection, or multiple organ failure. Variant syndromes (tuberculous lymphadenitis in children, severe systemic disease in persons with AIDS) are caused by organisms of the Mycobacterium avium-intracellulare complex (MAIC). The diagnosis of TB is based on tuberculin skin testing (negative in 20% of people with active TB), imaging studies (computed tomography is more sensitive than standard chest radiography in detecting pleural effusion, miliary disease, and cavitation), and the finding of the causative organism in sputum or tissue specimens by acid-fast or fluorochrome staining, nucleic acid amplificatin, or culture.

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