Every summer in the 1940s and 1950s, parents across America lived in fear of polio. The disease attacked the nervous system, and in severe cases it paralyzed muscles, making it impossible to walk or even breathe. Swimming pools, movie theaters, and playgrounds closed during outbreaks because nobody knew exactly how the virus spread. An American scientist named Jonas Salk spent years developing a vaccine to fight polio. He used a killed version of the virus, meaning the virus in the shot could not cause the disease but could teach the body to fight it. On March 26, 1953, Salk announced that his vaccine was safe and effective in human tests. Two years later, a massive trial involving 1.8 million children proved that the vaccine worked. When the results were announced on April 12, 1955, church bells rang across the country. Parents wept with relief. Schools held vaccination days, and children lined up to get their shots. Before the vaccine, the United States saw roughly 35,000 polio cases every year. By 1979, polio had been eliminated from the country entirely. Salk never patented his vaccine. When asked who owned it, he replied, "The people. Could you patent the sun?"