Tuberculosis, also called TB, is an infection caused by a bacteria. TB usually affects the lungs, but it can spread to the kidneys, bones, spine, brain, and other parts of the body. The most commonly used skin test to check for TB is the PPD — purified protein derivative. If you have a positive PPD, it means you have been exposed to a person who has tuberculosis and you are now infected with the bacteria (mycobacterium tuberculosis) that causes the disease. Not necessarily.