In 1930, following Charles Nicolle's 1909 discovery that lice were the vector of epidemic typhus, and following the work done on a vaccine for the closely related Rocky Mountain spotted fever, Weigl took the next step and developed a technique to produce a typhus vaccine by growing infected lice and crushing them into a vaccine paste. He discovered that a vaccine could be developed from lice stomachs' infected with "Rickettsia prowazeki", a harmful bacteria especially to humans"." He developed this first version of the vaccine in 1918 and began experimenting on guinea pigs and even human volunteers. He refined this technique over the years until 1933, when he performed large-scale testing in order to cultivate bacteria and experiment with the lice using a micro-infection strategy..