Tuberculosis (TB) Causes and Risk Factors

Tuberculosis facts

  • Tuberculosis (TB) is an infection, primarily in the lungs (a pneumonia), caused by bacteria called Mycobacterium tuberculosis. It is spread usually from person to person by breathing infected air during close contact.
  • TB can remain in an inactive (dormant) state for years without causing symptoms or spreading to other people.
  • When the immune system of a patient with dormant TB is weakened, the TB can become active (reactivate) and cause infection in the lungs or other parts of the body.
  • The risk factors for acquiring TB include close-contact situations, alcohol and IV drug abuse, and certain diseases (for example, diabetes, cancer, and HIV) and occupations (for example, health-care workers).
  • The most common symptoms and signs of TB are fatigue, fever, weight loss, coughing, hemoptysis, and night sweats.
  • The diagnosis of TB involves skin tests, chest X-rays, sputum analysis (smear and culture), and PCR tests to detect the genetic material of the causative bacteria.
  • Inactive tuberculosis may be treated with an antibiotic, isoniazid (INH), to prevent the TB infection from becoming active.
  • Active TB is treated, usually successfully, with INH in combination with one or more of several drugs, including rifampin (Rifadin), ethambutol (Myambutol), pyrazinamide, and streptomycin.
  • Drug-resistant TB is a serious, as yet unsolved, public-health problem, especially in Southeast Asia, the countries of the former Soviet Union, Africa, and in prison populations. Poor patient compliance, lack of detection of resistant strains, and unavailable therapy are key reasons for the development of drug-resistant TB.
  • The occurrence of HIV has been responsible for an increased frequency of tuberculosis. Control of HIV in the future, however, should substantially decrease the frequency of TB.

What is tuberculosis?

Tuberculosis (TB) is an infectious disease caused by bacteria whose scientific name is Mycobacterium tuberculosis. It was first isolated in 1882 by a German physician named Robert Koch who received the Nobel Prize for this discovery. TB most commonly affects the lungs but also can involve almost any organ of the body. Many years ago, this disease was referred to as "consumption" because without effective treatment, these patients often would waste away. Today, of course, tuberculosis usually can be treated successfully with antibiotics.

There is also a group of organisms referred to as atypical tuberculosis. These involve other types of bacteria that are in the Mycobacterium family. Often, these organisms do not cause disease and are referred to as "colonizers" because they simply live alongside other bacteria in our bodies without causing damage. At times, these bacteria can cause an infection that is sometimes clinically like typical tuberculosis. When these atypical mycobacteria cause infection, they are often very difficult to cure. Often, drug therapy for these organisms must be administered for one and a half to two years and requires multiple medications.

How does a person get TB?

A person can become infected with tuberculosis bacteria when he or she inhales minute particles of infected sputum from the air. The bacteria get into the air when someone who has a tuberculosis lung infection coughs, sneezes, shouts, or spits (which is common in some cultures). People who are nearby can then possibly breathe the bacteria into their lungs. You don't get TB by just touching the clothes or shaking the hands of someone who is infected. Tuberculosis is spread (transmitted) primarily from person to person by breathing infected air during close contact.

There is a form of atypical tuberculosis, however, that is transmitted by drinking unpasteurized milk. Related bacteria, called Mycobacterium bovis, cause this form of TB. Previously, this type of bacteria was a major cause of TB in children, but it rarely causes TB now since most milk is pasteurized (undergoes a heating process that kills the bacteria).

What happens to the body when a person gets TB?

When the inhaled tuberculosis bacteria enter the lungs, they can multiply and cause a local lung infection (pneumonia). The local lymph nodes associated with the lungs may also become involved with the infection and usually become enlarged. The hilar lymph nodes (the lymph nodes adjacent to the heart in the central part of the chest) are often involved.

In addition, TB can spread to other parts of the body. The body's immune (defense) system, however, can fight off the infection and stop the bacteria from spreading. The immune system does so ultimately by forming scar tissue around the TB bacteria and isolating it from the rest of the body. Tuberculosis that occurs after initial exposure to the bacteria is often referred to as primary TB. If the body is able to form scar tissue (fibrosis) around the TB bacteria, then the infection is contained in an inactive state. Such an individual typically has no symptoms and cannot spread TB to other people. The scar tissue and lymph nodes may eventually harden, like stone, due to the process of calcification of the scars (deposition of calcium from the bloodstream in the scar tissue). These scars often appear on X-rays and imaging studies like round marbles and are referred to as granulomas. If these scars do not show any evidence of calcium on X-ray, they can be difficult to distinguish from cancer.

Sometimes, however, the body's immune system becomes weakened, and the TB bacteria break through the scar tissue and can cause active disease, referred to as reactivation tuberculosis or secondary TB. For example, the immune system can be weakened by old age, the development of another infection or a cancer, or certain medications such as cortisone, anticancer drugs, or certain medications used to treat arthritis, inflammatory bowel disease, or organ transplantation. The breakthrough of bacteria can result in a recurrence of the pneumonia and a spread of TB to other locations in the body. The kidneys, bone, and lining of the brain and spinal cord (meninges) are the most common sites affected by the spread of TB beyond the lungs.

How common is TB, and who gets it?

Data from 2011 show that approximately 8.7 million new cases of TB occur each year worldwide, with 1.4 million estimated deaths. More disconcerting is the rise in multidrug-resistant organisms (MDR) accounting for 630,000 cases worldwide. Additionally, further mutation of the bacteria has resulted in even more difficult to treat infections referred to as extensively drug resistant (XDR) TB. XDR TB is now found in 84 different countries.

In the United States, it is estimated that 10-15 million people are infected with the TB bacteria, and 22,000 new cases of TB occur each year. The incidence of MDR TB and XDR TB in the U.S. is much lower than that seen worldwide, but the ease with which we travel necessitates vigilance to keep these numbers low.

Anyone can get TB, but certain people are at higher risk, including

  • people who live with individuals who have an active TB infection,
  • poor or homeless people,
  • foreign-born people from countries that have a high prevalence of TB,
  • nursing-home residents and prison inmates,
  • alcoholics and intravenous drug users,
  • people with diabetes, certain cancers, and HIV infection (the AIDS virus),
  • health-care workers.

There is no strong evidence for a genetically determined (inherited) susceptibility for TB.


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Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease


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