The Rats Were Framed!

4 min readJun 7, 2024

Well, Kind of. They Did Take Undo Blame.

The Traditional Story

You heard it in middle school. An army laid siege to Caffa in the Crimea and, reputedly, the defenders threw dead bodies over the wall. The fleas from the bodies hopped on the rats, the rats cuddled up with the humans, and pretty soon the ship bringing soldiers home to the Sicilian city of Messina was full of plague. The rats spread the fleas from the boats all over Europe and a third to half of everyone living in Eurasia at the time died.

Problems with the Story

The big problem was always speed: How did the Plague spread so far, so fast? Yersinia pestis takes effect quickly and has a high mortality rate, killing people within a few days. Even if you were of the 40 or so percent that survived the bubonic form (and virtually no one survived the pneumonic form), you were sick within one to seven days of contracting it. Sick people don’t travel far, and neither do rats. In fact, rats tend not to roam much at all, preferring to be within 300 feet of their home nests.

The other problem is that rats can suffer from Bubonic Plague. They can die of it. So…where were the little rat plague pits? Cats can also contract, spread, and die of it. Why did no one report a big die off of these animals before humans started suffering?

Another problem was that rats weren’t necessarily where plague was. Inland villages in Scandanavia didn’t have rats but were decimated by the Black Death.

Rats were also where the Black Death wasn’t. A historian named Micheal McCormick found archeological evidence that rattus rattus was in much of medieval Europe well before the 1300's.

Newer Evidence and Theories

The first thought was that it wasn’t Yersinia Pestis that people died of in the 1300’s. That it was pulmonary anthrax, or something that resembled Ebola.

This isn’t wild: the symptoms were not well translated because medieval folks had a different way of classifying and describing diseases than we do now. (Incidentally, it’s only we moderns who call it the Black Death or plague. Folks before the 1800’s called it the Great Mortality or just the Death.)

And even if they were describing it the same way, symptoms of plague are stomach pains and fever. Those symptoms happen with many diseases.

Or that it was a different Y. Pestis, which mutated the way our own Covid did. One writer pointed out that an outbreak of plague in Bombay in 1903 killed only 3% of its victims, but it killed between 30 and 60% of its victims in 1348. There are definitely signs that people could, with some of the outbreaks, develop a short-term immunity to it.

Or that the rats were unduly targeted. A study in Oslo put the blame on giant gerbils on the Silk Road.

Body Lice?

But a strong hypothesis about the spread of the Black Death puts the blame on those pesky upright apes and their personal parasites.

Body lice, lice that has been with we humans since the ice age, were initially thought to be inefficient vectors. But a new study has found this not to be so. The National Institute of Allergy and Infectious Diseases let body lice feed on blood that would be infected in the same amount that a sick human would and then monitored the viral load that the lice took on. Turns out that they became infected enough to routinely transmit the disease between humans when they feed.

That would fit what team at a university in Oslo found when they ran a computer model in 2018 of the spread of plague in 1348. They said at the time that the rapid jumps across cities better matched human and body lice transmission, though they preferred to blame gerbils.

Now leave poor little rattus rattus alone.