The bacterium Yersinia pestis infects about one in 1.6 million people every year. However, while it isn’t especially prevalent today, researchers say this tiny microbe changed the course of human history—not just once, but twice. Yersinia pestis caused an outbreak of bubonic plague in the year 541, 15 centuries ago, in the Byzantine Empire that contemporary observes said killed 10,000 people each day. Carried by rats that had stowed away on ships carrying imported grain, the Plague of Justinian, so called by historians after the emperor reigning at the time, spread from that empire’s cosmopolitan capital to much of Europe, Asia, and Africa, killing around half the population, weakening the empire militarily and politically, and possibly contributing to the Anglo-Saxon conquest of Britain. The illness recurred, in milder forms, for a further 200 years.
This plague was 800 years before the Black Death struck in 1347, but in 2010 scientists discovered that different strains of the same bacterium were responsible for both. In fact, analysis of remains from those eras suggests that the strain that caused the Plague of Justinian had died out by the year 1000. There are no traces of the Justinian strain of the microbe more recently than that; the Black Death was caused by a different type that survives to this day, though it has become much more rare. Medical historians used DNA recovered from the teeth of victims of the Justinian Plague in Bavaria, a region of Germany, to look for the microbe likely to be responsible for the ancient pandemic. Comparing it to known strains of Y. pestis, they saw distinct similarities, but also crucial differences, indicating the bacteria are related, but not identical.
Although plague is very rare in humans nowadays—though there was a third, smaller epidemic in the 19th century in Asia and North America that resulted in the discovery of the bacterium responsible—it is still prevalent in rodent populations in certain parts of the world. Rodents of over 200 species that exist today have been found to be plague carriers. Fortunately, humanity has some things in our favor. People whose ancestors are from areas where a plague epidemic lasted a long time may have inherited a degree of evolved immunity. The Plague of Justinian lasted long enough that people from Europe and the Middle East, and their descendants, have a degree of natural resistance. Better sanitation than was standard 800 and 1600 years ago means humans are seldom infected by rats. In addition, plague bacteria have not developed antibiotic resistance.