When I was in Japan - '53 to '55 - a scary mystery disease was known to be sickening/killing GIs in Korea. Twas Hanta. Been causing humans problems in, at least, Korea for a very long time.
Viruses will apparently often just mutate and jump species. This happened in the early 90s with hantavirus. Suddenly people were dying from it, yet it was known only as a rodent virus. But not all strains could jump to humans, and only a relatively few people have gotten it.