Tuberculosis is a disease that can affect any part of the body, but the infection most often targets the lungs. Most TB cases are caused by inhaling the bacterium M. tuberculosis. It is also possible to contract TB by drinking unpasteurized milk. People can contract TB when an infected person coughs, sneezes, speaks, sings, or laughs, the National Institute of Allergy and Infectious Diseases (NIAID) says. Although tuberculosis has been around for thousands of years, it did not become a major health problem until the Industrial Revolution, when crowded living conditions helped it spread. In the 17th and 18th centuries, TB caused a quarter of all adult deaths in Europe. Only the active form of the disease is contagious. Some people have inactive TB, or latent TB, which cannot be spread to other people unless it becomes activated for some reason.

TB is especially prevalent among people with HIV. Other groups at risk for TB include the urban poor, intravenous drug users, the homeless, prison inmates, and health care workers.

Most other infectious diseases progress at a fast clip, with microbes multiplying quickly and causing an onslaught of symptoms. The M. tuberculosis bacterium is a slow multiplier. It takes two to eight weeks after a person is infected with TB for enough bacteria to exist for the immune system to respond. Most people heal completely after that initial infection and never develop an active form of TB. They have what is called latent TB infection. They are not contagious and cannot pass TB on to others. In people who do develop active (infectious) TB, it may take several decades for that to occur. The active form may progress more quickly in people who have weak immune systems.

These are late symptoms of active TB. Early symptoms may be so vague—a fever and a cough—that they are not noticed. The early symptoms then ease, and no other symptoms are present until much later. Besides weight loss, night sweats, and loss of appetite, another late symptom is a cough. If active TB is not treated, one in three patients will die within several months, the NIAID says. Those who don't die can face a chronic form of active TB that leads to coughing and chest pain.

The first step in diagnosing TB is a skin test, which will show whether a person has been exposed to M. tuberculosis. A small amount of purified protein derivative is injected beneath the skin. This test will not show a positive reaction until at least three weeks after a TB exposure or primary infection. A positive skin test does not mean the person has active TB, however; it simply means the person was exposed to the bacterium at some point. A blood test called the QuantiFERON-TB Gold test now also is available in some health departments to check for TB exposure. Besides the skin or blood test, the doctor will look at the person's medical history and X-ray of the lungs to check for signs of active or old/healed tuberculosis. The doctor may also take samples of morning sputum and stain it to check for evidence of the TB bacterium. The sputum may also be sent for culture of the M. tuberculosis bacteria, although this may take several weeks to show a positive result.

Before antibiotics became available, half of people with active TB died within two years. Now, antibiotics can cure nine out of 10 patients with active TB, the NIAID says. Treatment usually combines several antibiotics, given for six to 12 months. It is important that patients take their medication on time every day. When patients stop taking their medication before the end of treatment they risk having TB return. TB that returns may be resistant to drugs used to treat it, making it much more difficult to cure. If you have a positive skin test for TB, but no other signs of the disease, your doctor may ask you to take antibiotics for nine months to kill any TB bacteria in your body and prevent development of active TB in the future.

The number of TB cases began dropping in the 1940s and 1950s, when antibiotics were first used to treat the disease. But after 1985, the number of TB cases started to rise again. One factor was the AIDS epidemic, the NIAID says. People with HIV infection are at high risk for developing active TB. People who must live in crowded homeless shelters are at higher risk for developing active TB. People who don't finish their TB treatment are at risk for developing drug-resistant TB.

Although a vaccine is available, it is not recommended in this country because the rate of TB infection is low and because vaccinating people would make the TB skin test less useful. Someone vaccinated against TB may show a positive skin test even if not infected, especially in the few years following vaccination. The World Health Organization recommends that infants who live in areas of the world where TB is prevalent be vaccinated against TB. The vaccine does not work very well in adults, the NIAID says.