(CNN)Plague outbreaks in London spread four times faster in the 17th century than they did in the 14th century, researchers have estimated after studying troves of wills and death records.
Plague history shows how a pandemic's course can be shaped
Researchers from Canada's McMaster University analyzed thousands of documents spanning 300 years -- including personal wills and testaments, parish registers and the London Bills of Mortality -- to search for patterns on how plague was spreading through the population.
Plague, one of the deadliest bacterial infections in human history, caused an estimated 50 million deaths in Europe during the Middle Ages when it was known as the Black Death. The disease, though rare and now treatable with antibiotics, is still around today -- cases have been recorded in China and the United States as recently as this year.
There was a "striking acceleration" in plague transmission between the Black Death of 1348 and the Great Plague of 1665, researchers said in findings published Monday in the Proceedings of the National Academy of Sciences journal.
David Earn, a professor in the department of mathematics and statistics at McMaster and lead author of the research, told CNN that while plague cases in London doubled every six weeks in the 14th century, by the 17th century, they were doubling every week and a half.
"That's an enormous difference," he said.
But this was not simply a case of the disease becoming more virulent -- evolutionary geneticist Hendrik Poinar told CNN that while the spread of the disease accelerated, genetic analysis to date tentatively suggests that it may have become less infectious.
"When there are shifts in the epidemiology of the disease, most of the shifts that occur can be translated to human intervention o