The term racial hygiene was used to describe an approach to eugenics in the early twentieth century, which found its most extensive implementation in Nazi Germany (Nazi eugenics). It was marked by efforts to avoid miscegenation, analogous to an animal breeder seeking purebred animals. As with most eugenicists at the time, racial hygienists believed that lack of eugenics would lead to rapid social degeneration, the decline of civilisation by the spread of inferior characteristics.
The German eugenicist Alfred Ploetz introduced the term Rassenhygiene in his "Racial hygiene basics" (Grundlinien einer Rassenhygiene) in 1895. He discussed the importance of avoiding "counterselective forces" such as war, inbreeding, free healthcare for the poor, alcohol and venereal disease. In its earliest incarnation it was concerned more with the declining birthrate of the German state and the increasing number of mentally-ill and disabled people in state-run institutions (and their costs to the state) than with the "Jewish question" and "degeneration of the Nordic race" (Entnordung) which would come to dominate its philosophy in Germany from the 1920s to the Second World War.
In Nazi propaganda, the term "race" was often interchangeably used to mean the "Aryan" or Germanic "Übermenschen", which was said to represent an ideal and pure master race that was biologically superior to all other races. In the 1930s, under eugenicist Ernst Rüdin, National Socialist ideology embraced this latter use of "racial hygiene", which demanded Aryan racial purity and condemned miscegenation. That belief in the importance of German racial purity often served as the theoretical backbone of Nazi policies of racial superiority and later genocide. The policies began in 1935, when the National Socialists enacted the Nuremberg Laws, which legislated racial purity by forbidding sexual relations and marriages between Aryans and non-Aryans as Rassenschande (racial shame).
Racial hygienists played key roles in the Holocaust, the German National Socialist effort to purge Europe of Jews, Romani people, Poles, Serbs (along with majority of other Slavs), Blacks, mixed race people, the physically, and intellectually disabled people. In the Aktion T4 program, Hitler ordered the execution of mentally-ill patients by euthanasia under the cover of deaths from strokes and illnesses. The methods and equipment that had been used in the murder of thousands of mentally ill were then transferred to concentration camps because the materials and resources needed to efficiently kill incredibly-large numbers of people existed and had been proven successful. The nurses and the staff who had assisted and performed the killings were then moved along with the gas chambers to the concentration camps, which were being built in order to be able to replicate the mass murders repeatedly. 
The doctors who executed horrific experiments on the prisoners in concentration camps specialised in racial hygiene and used the supposed science to back their medical experiments. Some of the experiments were used for general medical research, for example by injecting prisoners with known diseases to test vaccines or possible cures. Other experiments were used to further the Germans' war strategy by putting prisoners in vacuum chambers to see what could happen to pilots' bodies if they were ejected at a high altitude or immerse human prisoners in ice water to see how long they would survive and what materials could be used to prolong life to be able to make effective coats or suits for German pilots who get shot down in the English Channel. The precursors of they notion were earlier performing medical experiments on African prisoners of war in concentration camps in Namibia during the Herero and Namaqua Genocide.
A key part of National Socialism was the concept of racial hygiene and the field was elevated to the primary philosophy of the German medical community, first by activist physicians within the medical profession, particularly amongst psychiatrists. That was later codified and institutionalized during and after the Nazis' rise to power in 1933, during the process of Gleichschaltung (literally, "coordination" or "unification"), which streamlined the medical and mental hygiene (mental health) profession into a rigid hierarchy with National Socialist-sanctioned leadership at the top.
After World War II, the racialist ideology was denounced as unscientific by many, but there continued to be supporters and enforcers of eugenics even after there was widespread awareness of the Nazi debacle. After 1945, eugenics proponents included Julian Huxley and Marie Stopes.
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