In the first longitudinal study of its kind, focusing on a single region for 800 years (between 1000 and 1800AD), researchers reconstructed Yersinia pestis genomes, the bacterium responsible for the plague, and showed that it was reintroduced into the Danish population from other parts of Europe again and again, perhaps via human movement, with devastating effects.

The historical samples were taken from nearly 300 individuals located at 13 different archaeological sites throughout the country.

"We know that plague outbreaks across Europe continued in waves for approximately 500 years, but very little about its spread throughout Denmark is documented in historical archives," says Ravneet Sidhu, one of the study's lead authors and a graduate student at McMaster's Ancient DNA Center, where the analysis was conducted.

The McMaster researchers, working with a team of historians and bioarchaeologists in Denmark and Manitoba, performed an in-depth examination of the relatedness and differences between the different strains of plague that were present in Denmark during this time.

They reconstructed and sequenced the genomes of Y. pestis, using fragments teased from ancient teeth, which can preserve traces of blood-borne infection for centuries. They compared the plague genomes to one another and to their modern-day relatives.

Researchers found positive plague samples in 13 individuals who had lived and died over a period of three centuries.