Spanish Flu -supposedly killed more than 100 million at only a 2.5% mortality rate-It was extremely contagious almost everyone in the entire world caught it. Ebola has never killed more than a few hundred at a time because it kills at a 90% plus mortality rate at about 7 days after exposure.

When some ancient diseases first hit humanity they were far deadlier than they are now. We've built up a little herd immunity on some of them. Some are well known some are almost mythical. It is said the great Mongolian hordes unleashed the plague on Europe after launching plague victims into a city in the Crimea. One of the first cases of biological warfare- read this http://wwwnc.cdc.gov/eid/article/8/9/01-0536_article.htm