An acute or chronic infection caused by Mycobacterium tuberculosis, tuberculosis (TB) is characterized by pulmonary infiltrates, formation of granulomas with caseation, fibrosis, and cavitation. People living in crowded, poorly ventilated conditions are most likely to become infected.
In patients with strains that are sensitive to the usual antitubercular agents, the prognosis is excellent with correct treatment. However, in those with strains that are resistant to two or more of the major antitubercular agents, mortality is 50%.
After exposure to M. tuberculosis, roughly 5% of infected people develop active TB within 1 year; in the remainder, microorganisms cause a latent infection. The host’s immune system usually controls the tubercle bacillus by killing it or walling it up in a tiny nodule (tubercle). However, the bacillus may lie dormant within the tubercle for years and later reactivate and spread. Those at higher risk for disease progression or reactivation of dormant disease include infants, the elderly, and individuals who are immunocompromised (such as those with acquired immunodeficiency syndrome [AIDS], those undergoing chemotherapy, or transplant recipients taking antirejection medications).
A person’s risk of contracting TB increases if he has frequent contact with people who have the disease, if he lives in crowded or unsanitary living conditions, or if he has poor nutrition. There has been an increase of incidence of TB in the United States due to the increase of human immunodeficiency virus infection, the increasing number of homeless individuals (poor environment and poor nutrition), and the appearance of drug-resistant strains of TB.
Reactivation risk factors
Although the lungs are the primary infection site, mycobacteria commonly exist in other parts of the body. A number of factors increase the risk of infection reactivation: gastrectomy, uncontrolled diabetes mellitus, Hodgkin’s disease, leukemia, silicosis, AIDS, and treatment with corticosteroids or immunosuppressants.
Mode of transmission
TB is transmitted by droplet nuclei produced when infected persons cough or sneeze. After inhalation, if a tubercle bacillus settles in an alveolus, infection occurs. Cell-mediated immunity to the mycobacteria, which develops about 3 to 6 weeks later, usually contains the infection and arrests the disease.