Murnau grew up with the Third Plague Pandemic scrabbling around the edges of his world; it mostly affected Asia, Australia, and the Americas, but it did spread west through Russia, although it doesn't seem to have affected the European continent much.

However, like every other adult in Europe in the 1920s, he had survived the 1918-1919 Influenza Pandemic. I can see that reminding people in Europe of the plague.