In 1915, the director of a veterinary hospital in London was trying to grow viruses in his lab. At the time, researchers had a general idea that viruses were some kind of ultra-small microbes. They caused diseases, but could pass through filters fine enough to remove all known bacteria. Unfortunately, nobody had figured out how to culture them. The veterinary hospital director, a physician named F. W. Twort, decided to use a brute-force approach.