in the 1930s, as germ theory developed, "the idea gained strength that organisms only stay healthy if they manage to keep out or otherwise defend themselves against all invaders [...] This idea not only led to large programs of hygiene that were meant to keep all people healthy. It was also applied to the peole, the population, as if this, like the individual, was an organism in its own right. The population--or the race, the words could be used interchangeably--should not be stained by foreign blood."
"racial hygiene became a meaningful concept"
see G. Stocking's Race, Culture, and Evolution