Turns out George Romero took the infectious aspect from the novel I Am Legend (made into Omega Man in 1971), which is a vampire story. He mashed up the European vampire with the Haitian "zonbi".

Some scholars think that the 18th century rabies epidemics shaped the vampire legend.

https://www.tampabay.com/archive/1999/04/20/rabies-seen-as-b...

Why does it being from 1968 mean it wasn’t inspired by rabies?

previous iterations were clearly not cool enough of a fantasy to be propagated