Polio has been around for thousands of years, but it had generally been a mild disease. When sanitation techniques improved at the end of the 19th century, people stopped getting the mild form of polio when they were babies. Instead, older children and adults got a more serious illness that was extremely contagious, and yet no one knew how it was transmitted.
Animal research was crucial to identifying what caused polio and finding a vaccine. In 1908 Drs. Karl Landsteiner and Erwin Popper proved that polio was an infectious disease by showing that monkeys injected with tissue from a person who died of polio would become ill with the disease. We now know that polio is transmitted when bodily wastes from a person who has the disease contaminate food or water, which then is ingested by another person.
Polio was difficult to stop because it is caused by a virus. Antibiotics such as penicillin were considered "wonder drugs" in the 1940s because they could cure bacterial infections, but they were useless against polio. Viruses are also harder to isolate than bacteria. In 1949 Drs. John Enders, Frederick Robbins, and Thomas Weller made a breakthrough when they figured out how to grow the polio virus in cell cultures. This achievement earned them a Nobel Prize.
During the 1950s, Drs. Jonas Salk and Albert Sabin developed two different polio vaccines. These vaccines "teach" the immune system how to defend itself against polio by exposure to the virus in a killed or weakened form. The vaccines were tested in animals to make sure that they were safe and effective before they were used in people.
Today polio has been virtually eradicated in the industrialized world, but it remains a problem in some developing countries.