Hilary Koprowski (1916-2013) began his work on poliovirus vaccine in the middle to late 1940s when he worked at Lederle Laboratories. He and his team attenuated Type 2 polioviruses by passing them through cotton rats, a living but unnatural host. Monkeys were used to test virulence. He and his technician drank a suspension of the cotton rat brain containing the attenuated virus to test for obvious toxicity. In 1950, Koprowski tested the vaccine in 20 institutionalized children and showed that it was effective in producing polio antibodies.