The most infamous doctor at Auschwitz was Josef Mengele, the "Angel of Death", who worked in Auschwitz II from 30 May 1943, at first in the gypsy family camp. Particularly interested in performing research on identical twins, dwarfs, and those with hereditary disease, Mengele set up a kindergarten in barracks 29 and 31 for children he was experimenting on, and for all Romani children under six, where they were given better food rations. From May 1944, he would select twins and dwarfs during selection on the Judenrampe, reportedly calling for twins with "Zwillinge heraus!" ("twins step forward!"). He and other doctors (the latter prisoners) would measure the twins' body parts, photograph them, and subject them to dental, sight and hearing tests, x-rays, blood tests, surgery, and blood transfusions between them. Then he would have them killed and dissected. Kurt Heissmeyer, another German doctor and SS officer, took 20 Jewish children from Auschwitz to use in pseudoscientific medical experiments at the Neuengamme concentration camp. In April 1945, the children were killed by hanging to conceal the project.
Nazi Germany maintained concentration camps (German: Konzentrationslager, KZ or KL) throughout the territories it controlled before and during the Second World War. The first Nazi camps were erected in Germany in March 1933 immediately after Hitler became Chancellor and his Nazi Party was given control of the police by Reich Interior Minister Wilhelm Frick and Prussian Acting Interior Minister Hermann Göring. Used to hold and torture political opponents and union organizers, the camps initially held around 45,000 prisoners. In 1933–1939, before the onset of war, most prisoners consisted of German Communists, Socialists, Social Democrats, Roma, Jehovah's Witnesses, homosexuals, and persons accused of 'asocial' or socially 'deviant' behavior by the Germans.