Jonas Salk was an American medical researcher and virologist who developed the first successful polio vaccine. He is widely credited with conquering polio, a highly contagious and potentially fatal disease that had been a major health threat in the United States and around the world. Salk's vaccine was approved for use in 1955 and within a few years, polio had been virtually eliminated in the United States. His work has saved millions of lives and continues to be a major influence in the fight against infectious diseases.