On March 26, 1953, Jonas Salk announced a discovery that would save hundreds of thousands of lives a year. The scientist had created the world’s first vaccine against polio. A vaccine is a medicine that protects people from contracting a specific disease. “Salk became a celebrity overnight,” says Dr. Howard Markel, a medical historian at the University of Michigan. Polio affects mostly children, causing paralysis, or inability to move, and even death. It was one of the most feared illnesses of the early 20th century. Today, polio has almost been wiped out. To celebrate Salk’s accomplishment, World Polio Day is held every 24th of October—the same month Salk was born.