There are a lot of unexplained mysteries about the Black Death. One mystery is why the plague might kill millions and then vanish, only to reappear years later.

Throughout history, few diseases have captured the imagination as much as the Black Death, a bacterial illness spread by flea bites which caused a number of major pandemics across Europe and Asia in the last thousand years. To this day, there are a lot of unexplained mysteries about the Black Death, including precisely what bacteria it was – the best evidence is that it was Yersinia pestis, the agent that causes bubonic and pneumonic plague, but we aren’t sure – and how it spread the way that it did. One mystery is why the plague seemed to strike in waves; an outbreak might kill millions, and then it would vanish, only to reappear years later. This is a useful question for us to understand today for two reasons: first, because in the modern era we’re working very hard to eliminate diseases, so we need to know how they hide from us, and second, because it’s handy for a storyteller to know what things might trigger a plague in case they want those circumstances to happen in a game.

Broadly speaking, there’s one main way that diseases hide from us in nature: reservoirs, which usually means animals who can carry but not necessarily be affected by the disease, or at least, not be wiped out by it. If the plague was able to remain in rodent reservoirs, for example, then it would make sense that even if humans manage to eliminate it in their cities, it might still reappear years later when the wrong rat happens to find its way into the wrong house. There’s evidence that this is what happened in some parts of Asia, where gerbils have been implicated as a major spreader of fleas during the plague years. Studies have shown that in some of the Slavic regions, plague deaths waxed and waned along with the apparent size of the gerbil population. In contrast, despite many years of searching, no European reservoir of the Black Death has ever been identified, so there’s long been a lot of curiosity about what determined when and where the plague would reappear. In just the last few years, there’s been some suggestion that climate changes from year to year might have led to the disease coming and going from Europe, which would be an interesting example of how just a few degrees difference in the temperature can affect the lives of millions.

Does climate impact disease spread? In some ways, unquestionably, but in the case of the Black Death, there’s some evidence so. In the case of the Slavic gerbils, for example, the thought is that because their populations swelled to massive sizes during particularly warm and wet seasons, some years would have seen them (and their fleas) spilling into human towns. In contrast, cooler and drier years would have shrunk the gerbil population, which might have forced fleas to leave their usual hosts and seek out handy alternatives, such as humans. Either one of these circumstances could have resulted in a resurgence of pestilence: the plague would spread between cities in hot years and within cities in cold years, and over a decade might cover an entire country.

It turns out that when we look at tree rings, as a way of measuring what the climate was like in a region from year to year, there’s an important association with some otherwise unexplained outbreaks in Europe. Like the proverbial butterfly of Edward Lorenz, flapping its wings and causing a hurricane, we can see that a year of particularly good growth for trees in Karakorum predicts an outbreak of plague in Europe roughly fifteen years later. A hot wet summer in Pakistan leads to a boom in rodent population; a boom of rodents allows the flea population to explode; and over ten years, with and without the help of those pesky human merchants (and merchants’ camels, a favourite food of fleas), the rodents and their descendants carry a nightmarishly huge dose of Yersinia pestis across thousands of miles to Europe (causing outbreaks the whole way across Asia, naturally… they didn’t just magically appear in England).

These climate changes would explain, not merely why the plague would reappear in a region, but why it might reappear all across an entire region at once, instead of in the smaller and more discrete area you might expect from a single infected individual arriving and coughing in the street. Changes in a climate, after all, can’t help but affect a wide area all at once.

Above and beyond this, of course, there are plenty of other ways for the plague to be reintroduced to Europe from time to time. For all the evidence that climate changes played a role, this was probably a much less important factor than accidental importation of plague rats from ships  arriving in harbour. When you get right down to it, humans tend to be the main reason why most plagues spread among humans. Even as self-destructive as humans can be, they’re rarely the only problem.

Would the same mechanisms play an important role in a fantasy setting? Diseases would have far more effective ways to travel in a magical world, from rats hiding in airships and teleported merchandise to disgruntled druids summoning the plague to devastate eco-unfriendly cities. Still, it’s likely that some of the same mechanisms would play a role, and above all it reminds us that the long-term effects of small things can have a big impact twenty years down the road. Every time a player casts control weather or blows up a dam, there might always be some long-term consequences which could set up another story. 

More than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on August 23, 2015. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.