Tuberculosis is usually of a low order of infectivity. Once treatment has begun, patients are no longer infectious to close contacts, and there is no benefit to isolating them. Among the risk factors associated with tuberculosis that reactivates after many years of dormant infection, the coexistence of silicosis, diabetes mellitus, and the postgastrectomy state with tuberculosis are reasonably well demonstrated. Preventive treatment begins with prompt institution of chemotherapy in the index case. Isoniazid is extremely effective in preventing tuberculosis infection from becoming tuberculosis disease. The benefits of BCG vaccine are controversial, and it is little used in the United States. Hepatotoxicity is a potential serious side effect of isoniazid chemoprophylaxis. Clinical monitoring for prodromal symptoms makes the drug safe and effective for patients under 35 years of age.