In Vienna, Karl Landsteiner, MD (1868-1943), and Erwin Popper, MD (1879-1955), announced that the infectious agent in polio was a virus.
Popper and Landsteiner deduced the viral nature of polio by carefully filtering preparations of spinal cord fluid from a person who had died of polio. The filters were known to trap bacteria. When Popper and Landsteiner injected the filtered preparations into monkeys, the monkeys developed polio. The researchers then concluded that an infectious particle smaller than bacteria caused the disease.