 The Buchenwald concentration camp was liberated in April 1945. One month later Fleck found his wife and son in Lvov, which once again belonged to the Soviet Union. They hastily left for Poland, whose territory—by the decision of the Allies—was moved 200 km to the west. The Communist government in Poland was establishing Maria Curie-Skłodowska University in Lublin, and since Germans murdered many Polish scholars, there was a scarcity of employable professors. In October 1945 Fleck became the head of the Institute of Medical Microbiology at that university, and he became a full professor in 1950. In 1948 he testified in the Nuremberg Trials of German doctors, who conducted criminal experiments on prisoners of concentration camps.