The Black Death

Here are four rats rowing a tiny boat, painted in a 14th-century French book. Such an image calls to mind the rats that spread the plague bacterium Yersinia pestis during the worst pandemic in human history: the Black Death. The theory goes that rats carrying the infected fleas spread the bubonic plague that ended up killing over a third of Europeans in only a couple of years.

However, a mathematical modeling study published in 2018 suggests that rats might have played a far less important role than previously thought.

The plague can spread through airborne transmission of infectious droplets — like, someone with the plague coughs. But the more infamous and typical transmission is through the bite of fleas (or lice) who carry the disease. That’s when victims get the “buboes” that give their name to the Bubonic plague — swollen infected lymph nodes.

Studies of the Black Death in the 14th-century assumed that rats were the major European transmitters of these fleas: that tends to be the case today where the plague still exists (except in the Great Plains of the U.S., where prairie dogs and black-footed ferrets are carriers).

Computational mathematicians running simulations of mortality rates from the Black Death have suggested another route — human transmission. Although the bubonic plague is a zoomorphic disease (ie, spreading from non-human animals), human lice and human-based fleas fit the pattern in seven out of nine cities modeled in the simulation. Arguing that rat-to-human transmission wouldn’t have spread the disease as quickly, these epidemiologists think humans were the likeliest carriers because all of Western Europe was hit in the few years after a boat carrying infected people landed in the Italian city-states of Genoa in 1347.