During the Nazi German occupation of Poland in World War II, Weigl's research attracted the attention of the Nazis. When they occupied Lviv, they ordered him to set up a typhus vaccine production plant at his Institute. Weigl hired a number of Jewish friends and colleges at the production plant. About a thousand people worked there. Weigl employed and protected Polish intellectuals, Jews and members of the Polish underground. Many of these people he hired assisted him in his typhus research and experiments with lice. Many his Jewish associates helped primarily help feed the lice and in return, they received food, protection, and doses of the vaccine when it was fully developed. His vaccines were smuggled into ghettos in Lviv and Warsaw, various concentration camps, and even certain Gestapo prisons. It was estimated that Weigl was able to save around 5,000 lives during the Nazi's reign. His institute was later shut down by the Soviet Union following their 1944 anti-German offensive.