Tuberculosis (TB) is a life-threatening infection that primarily affects your lungs. Every year, tuberculosis kills nearly 2 million people worldwide. The infection is common - about one-third of the human population is infected with TB, with one new infection occurring every second. To check patients for the infection a simple skin test is conducted. For the skin test, a small amount of a substance called PPD tuberculin is injected within the skin of your inside forearm. You should feel only a slight needle prick. Within 48 to 72 hours, a health care professional will check your arm for a local reaction to the injected material.