Comment: Or maybe they were brought in on cosmic bodies such as comets. Learn more: The Apocalypse: Comets, Asteroids and Cyclical Catastrophes by Laura Knight-Jadczyk
Dr. George Poinar Jr., an entomologist at Oregon State University in the United States has possibly identified a previously unknown strain of the plague bacteria. The fossilized bacteria are preserved on a flea encased in amber. If the bacteria do belong to the bubonic plague, then at 20 million years, they are the oldest known evidence for that disease.
Poinar, who specializes in specimens caught in amber, says if the fossilized bacteria are related to the plague's Yersinia pestis bug, the disease predates the human race and traveled around much of the world.