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.