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 bacteria. People can get TB when an infected person coughs, sneezes, speaks, sings, or laughs. Tuberculosis has been around for thousands of years, but it did not become a major health problem until the Industrial Revolution. 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. This can't be spread to other people unless it becomes activated for some reason.

TB is especially common in people with HIV. Latent or inactive TB is more likely to become an active TB infection in someone with HIV. Other groups at risk for TB include people taking TNF antagonist medicines. These are medicines that suppress the immune system and are used to treat rheumatoid arthritis, inflammatory bowel disease and lupus. Others are risk include the urban poor who live in crowded areas with little access to health care, IV drug users, homeless people, prison inmates, people born in certain areas overseas, and health care workers.

Many infectious diseases cause illness very quickly. With TB, a person can be infected with the bacteria, but not become sick with TB disease. This is called latent TB infection. A person with latent TB is not contagious and cannot pass TB on to others. The other type of TB is active TB disease. Active TB may take several decades to occur—or it may develop soon after infection. Someone with active TB is contagious and can spread the disease to others. The active form may progress more quickly in people who have a weak immune system. Other infectious diseases have a latent period. These diseases can be hard to diagnose.

Symptoms of active TB can depend on what part of the body has been infected. If active TB occurs in the lungs, the symptoms can include a cough lasting 3 weeks or more, pain in the chest area, a cough that brings blood or mucus, chills, fever, and fatigue. TB can also affect a person’s brain, bones, kidneys, or spine.

The first step in diagnosing TB is a skin test. This will show whether you have been exposed to the bacteria. For the skin test, or PPD, a substance called tuberculin is injected just under the skin on your forearm. After 72 hours, you return to your healthcare provider, who checks the site for a positive or negative reaction. A positive skin test does not mean you have active TB. It simply means that you were exposed to the bacterium at some point or may have received the TB vaccine, called BCG. This vaccine is often routinely given in other countries, although it is not used in the U.S. A blood test called a quantiferon gold or IGRA is available in some health departments to check for TB exposure. Besides the skin or blood test, your doctor will look at your health 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 TB bacteria. The sputum may also be sent for culture of the bacteria. This may take several weeks to show a positive result.

Treatment usually combines several antibiotics that you take for 6 to 12 months. It's important that you take your medicine on time every day. If you stop taking the medicine before the end of treatment, you risk having TB return. TB that returns may be resistant to medicines used to treat it. This makes 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 an antibiotic for 9 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. People with HIV infection are at high risk of developing active TB. People who must live in crowded homeless shelters are at higher risk of developing active TB. People who don't finish their TB treatment are at risk of developing medicine-resistant TB. People who are taking medicines that suppress their immune system are at risk for active tuberculosis.

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. Infants who live in areas of the world where TB is common should be vaccinated against TB. The vaccine does not work very well in adults.