A recent collaboration by the universities of Copenhagen, Denmark, and Cambridge in the UK have found some interesting new evidence that Yersinia pestis, long credited for three devastating pandemics, may have been hanging around in humans for a lot longer than previously estimated.

 Y. pestis was behind three waves of Bubonic plague, the first starting with the Plague of Justinian in 541 AD and continuing intermittently until 750 CE. The second was dubbed the "Black Death" and devastated Europe from 1347-1351 CE, although it continued sporadically until the 18th century. The third emerged in China in the 1850's and continued until the middle of the 20th century.

The international team of researchers sequenced the genome of bacteria found in the teeth of 101 adults dating from the Bronze age, from locations spanning across Europe. They found Y. pestis in seven samples. 

Of those seven, six lacked key genetic components that allow Y. pestis to use fleas as a vector from human infection.  A virulence gene called "ymt" and a mutation in an activator gene called "pla" work together, as ymt protects the bacteria from toxins in the flea's digestive tract, allowing it to build up in numbers. This chokes off the flea's digestive system, causing it to feel as though it's starving and will bite anything it can. The pla gene allows Y. pestis to spread across different tissues, so it is no longer limited to the pneumonic tissuebut can get into the blood and lymph nodes as well.

More than just an interesting historical fact, this study illustrates a concept that is particularly relevant today. Just as Y. pestis did, small scale diseases can flourish under the right conditions, and, with just the right mutations, can devastate populations.