Tuberculosis (TB) is an infection caused by the Mycobacterium tuberculosis bacteria. TB affects mainly the lungs. Other organs can be affected, sometimes at the beginning but usually in long-term infections. TB is a contagious disease (contact with aerosols caused by sneezing and spitting). Only some individuals infected with the bacteria will develop symptoms of the disease (active tuberculosis). The tuberculin skin test (TST) is a test that screens for individuals who have been exposed to the bacteria. The TST (purified protein derivative or PPD) is used to assess whether the person has developed an immune response to Mycobacterium tuberculosis by injecting a small amount of protein extracted from the bacteria (tuberculin) under the skin. The size of the skin reaction area is measured 48 hours after injection. In healthy individuals, a reaction area greater than 10 mm confirms the probable presence of the bacteria. A smaller area (4 or 5 mm) is considered positive for certain categories of people. Negative results are seen in about 15% to 20% of carriers of the bacteria. A negative result therefore does not completely rule out the possibility of a latent or active Mycobacterium tuberculosis infection.