In 1908, Karl Landsteiner and Erwin Popper showed that the causative agent of poliomyelitis could be transmitted to monkeys by injecting into them material prepared by grinding up the spinal cords of children who had died from the disease. This proved that polio was caused by a virus.

These findings were confirmed the following year by Simon Flexner and Paul Lewis, laying the foundations of our knowledge of poliomyelitis.