 Twort and his brother, Dr. C. C. Twort, had for some years been trying to grow viruses in artificial media hoping to find a nonpathogenic virus, which might be the wild type of a pathogenic one, so more likely to grow.In 1914, Twort set out to identify the elusive (now known to be nonexistent) "essential substance" that would allow vaccinia virus to grow "in vitro".At the time, smallpox vaccines had to be made in the skin of calves and was almost always contaminated with the bacterial genus "Staphylococcus".