From the time of the Black Death, which happened around 1348, and the Great Plague of 1665, there were several epidemics of plague that occurred in Europe. Researchers analyzed the transmission of the disease over that period, and determined that in the 14th century, when around a third of Europe's population was killed by the plague, the number of infected people doubled about every 43 days. However, during later epidemics and the Great Plague, the number of infected individuals doubled about every eleven days - a dramatic increase in the rate of infection. The findings have been reported in the Proceedings of the National Academy of Sciences.
"It is an astounding difference in how fast plague epidemics grew," said the lead study author David Earn, a professor in the Department of Mathematics & Statistics at McMaster University.
Public death records for London didn't start to be collected until 1538. So in this work, Earn and an investigative team that included statisticians, biologists, and geneticists studied historical documents like personal wills and the London Bills of Mortality to estimate death rates.
"At that time, people typically wrote wills because they were dying or they feared they might die imminently, so we hypothesized that the dates of wills would be a good proxy for the spread of fear, and of death itself. For the 17th century, when both wills and mortality were recorded, we compared what we can infer from each source, and we found the same growth rates," explained Earn.
"No one living in London in the 14th or 17th century could have imagined how these records might be used hundreds of years later to understand the spread of disease," added Earn.
We know that three types of plague, pneumonic, septicemic, and bubonic, are caused by a pathogenic microbe called Yersinia pestis.
"From genetic evidence, we have good reason to believe that the strains of bacterium responsible for plague changed very little over this time period, so this is a fascinating result," noted study co-author Hendrik Poinar, a professor in the Department of Anthropology at McMaster.
The study authors suggested that many of the epidemics they studied probably did not spread primarily through human to human contact. Instead, early and late pandemics seem to have been related to the spread of infected fleas, which are known to spread bubonic plague. Other factors like living conditions, cold temperatures, and population density were probably contributing factors, the researchers noted. If we can learn from these patterns we might be in a better position to respond to COVID-19 and other pandemics that might occur in the future.