This information was reviewed and approved by
Michael D. Iseman, MD (1/31/2013).
Tuberculosis (TB) is an infectious disease caused by a germ, or bacterium, called Mycobacterium tuberculosis. This germ often affects the lungs; however, it may involve any organ and may infect anyone at any age. It is resistant to most antibiotics and thus difficult to treat. TB is spread from person to person, usually through the air, when a person with active disease coughs and sprays the bacteria into the air.
TB is not easily spread and typically involves weeks of indoor contact with a person who is infected with TB.
Left untreated,TB can kill approximately one half of patients within five years and produce significant morbidity (illness) in others. Inadequate therapy for TB can lead to drug-resistant strains of M. tuberculosis that are even more difficult to treat.
Not everyone who inhales the germ develops active TB disease. In most tuberculosis infections, the body's natural defenses are able to control the infection. Only about 10 percent of those infected will develop active disease in their lifetimes. Instead, the bacteria persist as a latent TB infection, which cannot be spread to other people. Active disease can occur in an infected person when the body's resistance is low or if a large or prolonged exposure to the germs overcomes the body's natural defenses. The body's response to active TB infection produces inflammation that can damage the lungs. The damage can be extensive even if the symptoms are minimal.
Active TB disease occurs most often in the lungs. The lungs are where your body carries out gas exchange between the air and the bloodstream, exchanging carbon dioxide for fresh oxygen. This exchange takes place at the alveoli, tiny bubble-like divisions of the lungs. When you breathe, you inhale plenty of other material along with oxygen, including bacteria. In order to infect you, the tuberculosis bacteria have to pass through the defenses in your airway and reach the alveoli. However, 20% of cases reported annually in the United States involve other organs including, most commonly, lymph nodes, kidneys or bones including the spine.
When the body's immune system notices the bacteria, it surrounds them with immune cells, creating collections of cells known as granulomas and effectively cutting off the bacteria from the rest of the body. The bacteria can persist in granulomas for many years as a latent TB infection.
TB Case Rate in the U.S.
In the United States, TB is much less common than it used to be. Of the some 13,000 new cases of active disease each year in the United States, over half occur in persons born outside of the country. Tuberculosis is very common in the developing world. It has been estimated that as much as a third of the world's population is infected with M. tuberculosis, and worldwide about 1.6 million people die of TB every year. TB and HIV are closely associated; people with HIV are much more likely to develop active disease if they are infected with the bacteria that cause TB.
Since the introduction of effective antibiotics, tuberculosis management has changed dramatically. Most important, people with tuberculosis are no longer sent to specialized sanitariums; now, they are treated in general hospitals and clinics. Also, doctors now know that they can reliably prevent active disease among people with latent infections. However, misuse of drugs has led to the development of drug-resistant tuberculosis, which is harder to cure. If antibiotics don't work, TB can be deadly.