Tuberculosis (TB) is a contagious disease that usually affects the lungs, but can also spread to other parts of the body. It is caused by the airborne bacterium Myobacterium tuberculosis. Because it is airborne, TB is generally transmitted when someone infected with TB coughs, sneezes, laughs or otherwise expels breath into the air. Once in the air, the bacteria can stay alive for several hours. However, brief exposure to TB bacteria rarely results in infection. Most people who contract TB become infected after repeated exposure to the bacteria (e.g., from another household member).

In most cases, an infection causes no symptoms and the bacteria remain dormant in the body for many years. A person in this state is said to have latent TB. Latent TB occurs when the TB bacteria enter the body, but are attacked by a healthy immune system and neutralized when the bacteria reach the lungs. This usually happens within two to eight weeks of exposure to the bacteria. The bacteria remain alive in the body, but surrounded by a wall of white blood cells called macrophages, which create scab-like seals around the bacteria.

Latent TB produces no symptoms, is not contagious and, in 90 percent of cases, causes no problems for the infected person, according to the American Lung Association (ALA). However, latent TB can later reactivate if the immune system becomes compromised. When this happens, the macrophages begin to fail and the TB bacteria become active and begin to multiply.

Active TB (its disease state) produces symptoms and is highly contagious. When latent TB becomes reactivated, the TB bacteria form cavities in the lungs (pulmonary TB), where they continue to multiply and sometimes spread through the blood to other parts of the body, such as the kidneys, lymph nodes, reproductive system and the tissue surrounding the brain (extrapulmonary TB).

Roughly 10 percent of people with latent TB develop active TB, according to the ALA. Active TB is most likely to develop within the first year of infection by TB bacteria. This can happen for a variety of reasons, such as contracting a common illness (e.g., influenza or the common cold), taking certain medications or getting older, all of which may weaken the immune system. 

TB occurs most often among:

• Patients with HIV/AIDS or other illnesses that affect the immune system
  

  

• People born in a country with high TB rates
  
  
• Older adults
  
  
• People living in poor areas and who are medically underserviced
  
  
• Substance abusers

Cases of TB in children represent a small percentage of overall TB cases. Children usually contract TB from adults who have the disease. Additionally, there is a small risk that women with TB who are pregnant may pass the disease to their child. More commonly, babies born to infected mothers may be underweight and have other health problems. Because of this, pregnant women are often treated for latent TB. Although there is evidence that the antibiotics used to treat TB in pregnant women may cross the placenta, this does not seem to harm the fetus.

In the 1800s, many deaths worldwide were caused by TB. Since the 1940s, medical advancements have significantly reduced the prevalence of the disease in the United States and other developed countries. In the 1980s and early 1990s, there was a resurgence of TB cases in the United States. This was due in part to the AIDS epidemic, increased immigration from countries with high TB rates and higher instances of poverty and drug use, all of which are major risk factors for TB.

By the middle of the 1990s, national efforts to control the spread of TB resulted in a considerable drop in the number of reported cases, although the rate of decline has slowed in recent years. According to the U.S. Centers for Disease Control and Prevention (CDC), 10 million to 15 million people in the United States have latent TB, and over 14,000 cases of active TB were reported in 2005.