 This was not known until the virus was isolated in cultured cells and many people were shown to have had mild infections that did not lead to poliomyelitis.But, unlike other viral infections, the incidence of polio – the rarer severe form of the infection – increased in the 20th century and reached a peak around 1952.The invention of a cell culture system for growing the virus enabled Jonas Salk (1914–1995) to make an effective polio vaccine.