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If you’ve ever volunteered or worked in a hospital, nursing home or laboratory, you may remember having a tuberculosis (TB) skin test. But did you fully understand what TB is and why the tests are necessary? Though TB may not seem to be a major health concern in the U.S., this cunning disease remains a leading cause of death worldwide and a major risk to people with weak immune systems.

A strain of bacteria called Mycobacterium tuberculosis causes TB. People who are exposed to the bacteria may develop a form of the disease without actually being sick. This is called latent TB infection and means the body is able to fight the bacteria without spreading it to others. However, if the immune system is not in top-notch shape, the bacteria can multiply and develop into a dangerous active infection. Active TB infection is very serious and can be deadly if not treated.

An active TB infection can be spread easily, especially when the disease attacks the lungs (pulmonary TB). The bacteria spreads from one person to another through the air. When someone with active pulmonary TB coughs or even speaks, tiny droplets sent out from the lungs carry bacteria through the air. The droplets can easily be inhaled by others, especially in close contact. Common symptoms of pulmonary TB include chest pain, coughing—sometimes coughing up blood and mucus—and wheezing. People with active TB infection may also feel weak and have fever, chills or no appetite.

You might wonder what all this has to do with skin tests. Because active TB is so dangerous and because latent TB can develop into active disease, people who have a high risk of developing TB and their caregivers need to be tested for the disease. A small amount of solution containing an inactive part of the bacteria is injected under the skin. If the skin at the injection site doesn’t change, everything is fine. But if the skin becomes red or swollen, this indicates there are TB bacteria in the body and treatment is necessary.

Although the number of people infected in the U.S. has been steadily declining, more than 10 million people worldwide were sickened by TB in 2016. March 24 is World Tuberculosis Day. Learn more about how to stop the spread of TB.

Audrey A. Vasauskas, PhD, is an associate professor of physiology at the Alabama College of Osteopathic Medicine. She is a former volunteer editor for the I Spy Physiology blog.