  When Lvov was incorporated into the Ukrainian Soviet Republic in 1939, Fleck had been appointed the head of the municipal laboratory of hygiene and bacteriology. On June 30, 1941, Lvov was captured by Germans. Anti-Jewish pogroms began, and Germans also started to murder university professors of Polish origin. Fleck and his wife survived, probably because Rudolf Weigl included him on the list of employees of the Lvov Institute of Research on Typhus and Viruses. In August 1941, Germans resettled Fleck to the Lvov ghetto, where he developed and produced an anti-typhus vaccine from the urine of sick people. He was then relocated with his family to the area of the pharmaceutical company Laokoon and tasked with producing an anti-typhus vaccine for German soldiers. Finally he and his wife and son were deported to the concentration camp in Auschwitz. At first he was forced to do physical labor, but later he was transferred to the camp’s hospital with the assignment to conduct bacteriological tests on prisoners. In December 1943 Fleck was transferred to the concentration camp in Buchenwald. There a group of prisoners—scientists and medical doctors of various nationalities—led by a scientifically illiterate German doctor produced an anti-typhus vaccine.