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U.S. Department of Health and Human Services


Core Curriculum on Tuberculosis, 2000

Chapter 2
Transmission and Pathogenesis


TB is a communicable disease caused by Mycobacterium tuberculosis, or the tubercle bacillus. It is spread primarily by tiny airborne particles (droplet nuclei) expelled by a person who has infectious TB. If another person inhales air containing these droplet nuclei, transmission may occur. Some bacilli reach the alveoli, where they are ingested by macrophages. Infection begins with the multiplication of tubercle bacilli within these alveolar macrophages. Some of the bacilli spread through the bloodstream when the macrophages die; however, the immune system response usually contains the bacilli and prevents the development of disease. Persons who are infected but who do not have TB disease are asymptomatic and not infectious; such persons usually have a positive reaction to the tuberculin skin test. About 10% of infected persons will develop TB disease at some time in life, but the risk is considerably higher for persons who are immunosuppressed, especially those with HIV infection. Although the majority of TB cases are pulmonary, TB can occur in almost any anatomical site or as disseminated disease.


After working through this chapter, you will be able to

  • Describe how TB is spread;
  • List at least 10 conditions that increase the risk that TB infection will progress to TB disease;
  • Define primary and secondary drug resistance;
  • Describe the classification system for TB.

In the United States, the vast majority of TB cases are caused by Mycobacterium tuberculosis, sometimes referred to as the tubercle bacillus. M. tuberculosis and three very closely related mycobacterial species (M. bovis, M. africanum, and M. microti) can cause tuberculous disease, and they compose what is known as the M. tuberculosis complex. M. bovis and M. africanum are very rare causes of disease in the United States; M. microti does not cause disease in humans. Mycobacteria other than those comprising the M. tuberculosis complex are called nontuberculous mycobacteria. Nontuberculous mycobacteria may cause pulmonary disease resembling TB. 1


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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