On the way to work this morning listened to an NPR Morning Addition interview with David Quammen, author of "Spillover", a book that traces the cross over and evolution of diseases such as Ebola and HIV that cross over from animals to humans. As human populations over run the earth and spillover more and more into wild areas, more zoonotic diseases (diseases that cross from one species to another) will occur more frequently in human populations. When asked if Ebola is "the next big one" in terms of danger to human populations, he expressed that no, he doesn't think so, but it is a rehearsal for "the next big one."

The Bubonic plague, transmitted by fleas to rats to humans (actually responsible for 3 human plagues) was transmitted to human populations around the world in the 14th century. Probably first spread along the land route of the Silk Road from central Asia, and a little later by merchant ships. Estimates vary but it seems the plagues, caused by variants of the bacterium yersinia pestis, killed off anywhere from 30 to 60% of the world human population during the 14th century. The historian Barbara Tuchman estimated 1/3 of the population of Europe died from the 'Black Death during that century.

Here is a link to the Quammen interview this morning with a written synopsis.
http://www.npr.org/blogs/goatsandsoda/2014/08/05/338059797/the-ebola-outbreak-a-dress-rehearsal-for-the-next-big-one