Scientists from Germany have used a medieval mass grave to completely sequence the genome of the bacterium responsible for the Black Death. In the fourteenth century the Black Death swept through Europe and was responsible for killing between 30 and 50% of the population of Western Europe. Large cemeteries had to be built outside of the walls of London to cope with all of the deaths from the city. Scientists have used bones from these cemeteries to sequence the genome of Yersinia pestis the bacterium responsible for the Black Death. The scientists then compared this to the modern genome of Yersinia pestis and found the DNA to be almost completely identical. The mystery behind the Black Death remains as infection with the modern strain of Yersinia pestis is very different to that of the Black Death. The researchers speculated that the different presentation may have been due to Medieval European’s complete lack of exposure to this bacterium.