The Black Death, or Plague, which killed an estimated one-third of the European population in the 14th century, was not spread by rats, as previously thought, but by fleas and lice, according to a new study published in the Proceedings of the National Academy of Sciences.
“Plague is undeniably a disease of significant scientific, historic and pubic interest, and is still present in many parts of the world today,” the researchers write. “It is therefore crucial that we understand the full spectrum of capabilities that this versatile, pandemic disease has exhibited in the past.”
Bubonic plague is caused by the bacterium Yersinia pestis and is often spread through the bite of infected fleas, according to Live Science. Symptoms include fever, chills, and the telltale bubos, or painful, swollen lymph nodes, which give the disease its name.
Pneumonic plague, which can be spread from person to person by airborne droplets, is another version of the disease and is characterized by a severe lung infection.
The traditional explanation for the spread of the disease has been that after fleas fed on infected rats, they then jumped to human hosts. But the historical evidence does not support this mode of transmission, according to the authors.
For example, early historical records do not mention large numbers of dying rats, as do records from later European outbreaks in the 1800s. Plus, more people died from the Black Death and the disease spread faster and farther than do modern outbreaks.
The researchers used mathematical modeling to create different scenarios of plague transmission during Europe’s second pandemic. They found that spread by parasites best reflected death rates in seven out of nine regions.
“Overall, our results suggest that plague transmission in European epidemics occurred predominantly through parasites, rather than commensal rat or pneumonic transmission,” the authors write.