The cause of plague wasn't discovered until the most recent global outbreak, which started in China in 1855 and didn't officially end until 1959. The first breakthrough came in Hong Kong in 1894 when researchers isolated the rod-shaped bacillus responsible—Yersinia pestis. A few years later, in China, doctors noticed that rats showed very similar plague symptoms to people, and that human victims often had fleabites.

Plague is an acute infectious disease caused by the bacillus Yersinia pestis and is still endemic in indigenous rodent populations of South and North America, Africa and Central Asia. In epidemics plague is transmitted to humans by the bite of the Oriental or Indian rat flea and the human flea. The primary hosts of the fleas are the black urban rat and the brown sewer rat. Plague is also transmissible person to person when in its pneumonic form. Yersinia pestis is a very pathogenic organism to both humans and animals and before antibiotics had a very high mortality rate. Bubonic plague also has military significance and is listed by the Centers for Disease Control and Prevention as a Category A bioterrorism agent.1

The first well-documented pandemic was the Plague of Justinian, which began in 541 A.D. Named after the Byzantine emperor Justinian I, it killed up to 10,000 people a day in Constantinople (modern-day Istanbul, Turkey), according to ancient historians. Modern estimates suggest half of Europe's population was wiped out before the plague disappeared in the 700s.