Tuberculosis is caused by the airborne bacteria Mycobacterium tuberculosis.

It can also be caused by Mycobacterium bovis, which lives in animals and can be transmitted to children who drink unpasteurized milk from infected cows.

In developed countries, however, cattle are tested for tuberculosis, and most milk is pasteurized.

Mycobacterium tuberculosis is spread from person to person when someone with an active form of the disease emits tiny, bacteria-containing droplets into the air through coughing, sneezing, talking, singing, or laughing.

The bacteria can remain suspended in the air for hours, potentially infecting anyone who breathes them in.

Not everyone who inhales TB bacteria gets sick, however. Some people’s immune systems immediately kill the bacteria. In others, the bacteria remain in a latent, or dormant, state.

Developing active tuberculosis is much more likely if a person has a weakened immune system because of HIV or another reason. In addition, the rate of developing active tuberculosis is highest in the two years after initial infection (i.e., among those with a recently acquired infection).

Risk Factors for Tuberculosis

Risk factors for tuberculosis include anything that weakens a person’s immune system or puts someone in frequent, close contact with people who have active TB.

In the United States, some of the main risk factors for tuberculosis include:

  • Poverty
  • HIV infection
  • Homelessness
  • Being in jail or prison
  • Substance abuse
  • Having a weakened immune system for reasons other than HIV, such as chronic steroid use, patients on dialysis, and those who have had organ transplants and take drugs to prevent rejection

Preventing the Spread of Tuberculosis

To prevent the transmission of tuberculosis in healthcare settings, the Centers for Disease Control and Prevention (CDC) has issued guidelines that require most employees to be screened for tuberculosis upon being hired and subsequently on a regular, often annual, basis.

Some residential institutions, such as nursing homes, also screen all new residents for tuberculosis. Screening for active TB is best accomplished by a chest X-ray.

Some other steps toward preventing the spread of TB include:

  • Treating latent infection before it becomes active

Directly Observed Therapy


Studies show a higher cure rate among people receiving DOT than among those self-administering their TB drugs.


  • People with drug-resistant TB
  • People receiving intermittent therapy
  • Homeless or unstably housed people
  • People who abuse alcohol or illicit drugs
  • Children and adolescents