Tuberculosis is a bacterial infection that usually involves the lungs, although it can also involve other parts of the body. When a person inhales tuberculosis bacteria, an initial infection develops that does not cause disease. The bacteria can lie dormant in the body after this initial infection but cause disease years later, when the body is under stress, because of poor nutrition or disease. A person with active tuberculosis disease is contagious, meaning that the infection can spread from one person to another, almost always through coughing. Once a person is diagnosed with tuberculosis, treatment involves taking a combination of drugs, usually by mouth, for at least 6 months. Unfortunately, some forms of tuberculosis are not sensitive to the 2 usual drugs used to treat it (multidrug-resistant tuberculosis). Multidrug-resistant tuberculosis requires treatment with more and/or different drugs. When treating tuberculosis, doctors test the fluid patients cough up (sputum smear) to see whether it contains bacteria. A negative sputum smear means that the patient is no longer infectious. Usually these samples are also sent for culture growth, which often takes 8 to 10 weeks for a result. It is important to know when the tuberculosis bacteria no longer grow in culture because negative cultures mean that the disease is improving.