IIRC Reston virus, one of the forms of ebola, is more contagious and easier to spread than ebola typically is. However, it's only dangerous to some species of monkeys, although it can infect a wide range of animals. A decade ago, there was a suspected case of Reston virus being spread to a man via contact with pigs.
Now just imagine the Reston virus swapping genetic information with some other viruses (making it as lethal to humans as it is to monkeys), and then being spread to a farmer somewhere in Asia. And have this happen in the early-mid 20th century, when viruses are poorly understood and the world has no idea what ebola is (aside from a river in the Congo I guess). Maybe it wouldn't be as bad as the influenza pandemics of the 20th century, but a disease like this could kill millions of people globally in a pretty brutal fashion.