During the 17th century, London suffered a plague so severe that it killed almost 1/4 of its entire population in 18 months. Scientists haven’t been sure what exactly caused the widespread disease, until now. 
Up until recently, scientists have suspected that the 17th century plague was similar to that of the Black Death, but there was much debate about whether that was indeed the cause.
Researchers used the skeleton’s teeth to determine the bacteria they had been exposed to at the time of death. While bacteria cannot live for hundreds of years inside of teeth, their enamel shell can help preserve bacteria to allow for researchers to examine it.
The bacteria died shortly after the person carrying it did, meaning that it doesn’t pose a risk to scientists today. The DNA inside of the teeth was examined by Museum of London Archaeology (MOLA) and the Max Planck Institute in Germany to determine what exactly those who had perished had been exposed to.
Don Walker, senior osteologist at Museum of London Archaeology, said of modern samples of yersinia pestis:
“It does not behave that way today. It’s much slower and spreads less dramatically. Could it be that there was some form of mutation? Or was it to do with host susceptibility and response? Were humans carrying greater disease loads in those days (e.g., tuberculosis) and had poor nutrition, making them more vulnerable?”
Scientists will then compare this new DNA to DNA from the 14th century bubonic plague to attempt to source where the disease came from originally. Many theorize that the plague was brought over from Asia, though another popular theory remains that rodents contracted the disease and passed it on to humans.
Because of excavation for the planned Crossrail station, thousands of artifacts spanning over the past 70,000 years have been discovered. 
 The Independent