Infection with tuberculosis (TB) usually occurs when airborne droplets containing the bacterium Myobacterium tuberculosis are inhaled. These droplets may be present 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. Brief exposure to these bacteria rarely results in infection. Most people who contract TB become infected after repeated exposure to the bacteria (e.g., from another household member).

Anyone can become infected with TB. Because TB is caused by close contact with people who are already infected with TB, anyone who lives or works closely with someone with TB is at risk. However, the risk of contracting TB from contact with infected people is significantly increased among certain groups.

The most significant risk factor for TB is a weakened immune system. It is because of this that people with the human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS) are most at risk for TB. Both AIDS and the virus that causes it, HIV, damage the body’s immune system. This damage makes people with HIV or AIDS less able to fight infections, including TB.

People with HIV or AIDS are also more likely to experience complications. People with HIV are also more likely to develop TB in organs apart from the lungs (extrapulmonary TB). In addition, medications used to treat TB can interfere with those used to treat HIV and AIDS, making it difficult to effectively treat HIV/AIDS and TB. On a global scale, people with HIV are more likely to die from TB than any other disease. This is not the case in the United States, where TB is less prevalent.

However, TB is still a serious threat to people with HIV in the United States. According to the U.S. Centers for Disease Control and Prevention (CDC), 10 percent to 15 percent of all TB cases in the United States occur in people with HIV. Therefore, people who are diagnosed with TB are also advised to undergo testing for HIV.

Other factors which may increase a person’s risk of contracting TB, such as by adversely affecting the immune system, include:

• Autoimmune disorders such as diabetes and thyroid disease. Autoimmune disorders occur when the body attacks and damages its own tissue, mistaking the tissue as a threat. These conditions can suppress immunity, making it harder to fight a TB infection.
  
  
• Malnutrition. Failure to consume enough calories or maintain a balanced diet can impair the immune system. This is a particular risk for people who live in poverty.
  
  
• Substance abuse. The immune systems of people who are long-term abusers of illegal drugs or alcohol are often weakened due to long-term exposure to these substances, making them vulnerable to TB infection. Also, some TB medications can cause complications for those receiving methadone (a drug used to treat heroin addiction).
  
  
• Age. As the body ages, the immune system weakens. Additionally, older people are more likely to have been exposed to TB bacteria when young and have latent TB (dormant TB bacteria in the body), which may reactivate as the immune system weakens.
  
  
• Smoking. Recent studies have shown that smokers have an increased risk of developing both latent and active tuberculosis. According to these studies, a person who smokes may be as much as twice as likely to develop active tuberculosis than a non-smoker.

Another important risk factor for TB is a person’s country of birth. People who were born in a country that has a high incidence of TB such as some countries in Africa, Asia and Latin America account for over half the cases of TB in the United States, according to the CDC. People from these countries also have a higher rate of multidrug-resistant TB (MDR TB), a deadly strain of TB that is resistant to two or more of the most common TB medications, and extreme drug-resistant TB (XDR TB), a strain of TB that is resistant to most TB medications. This is largely because countries with high TB rates also tend to have poor health care and a lack of consistent treatment available for people with TB. Thus, the bacteria to which people from these areas of the world are exposed are more likely to be antibiotic-resistant.

People living or working in crowded conditions (e.g., in prisons, nursing homes, homeless shelters) also have an increased risk of TB. TB can easily spread in poorly ventilated areas where people spend large amounts of time together. Poverty increases the risk of living in a crowded environment. It also means these people would not be likely to receive adequate medical care.