Polio (poliomyelitis) is a highly infectious viral disease passed from person to person. It invades the nervous system and can paralyze a person within hours. Initial symptoms are fever, fatigue, headache, vomiting, and neck stiffness. Polio mainly affects children under 3. Before the polio vaccine was introduced in 1955, tens of thousands of U.S. children per year developed paralytic polio. Only the inactive polio vaccine (IPV), made from dead polio virus, is used in the U.S. Starting at 2 months of age, all U.S. children receive four doses of IPV. Follow the links below to find WebMD's comprehensive coverage of polio, its history, the risk it poses, preventive vaccines, and much more.