Black Death News

Mass grave of plague victims in southern France, from 1720-1721

The bubonic plague is all over the history and archaeology web sites, because of some recent publications that claim to make major advances in our understanding. The new work is certainly interesting, so let's have a look at it.

First, as I already noted here, modern genetic research has confirmed that both the Black Death of 1346-1350 and Justinian's Plauge of 541-542 were infections of Yersinia pestis. Certain skeptics had expressed doubt about this ever since Yersinia pestis was first identified as the plague organism, but this is now close to 100% certain.

Also, there has been a lot of genetic work on the evolution of Yersinia pestis over time. The most recent thinking is that the organism has been around for at least 8,000 years. This fits with archaeological data suggesting plague infections in eastern Europe and Ukraine by 4,000 BC. One recent study found that the dangerous variant that caused Justinian's Plague and the Black Death emerged around 2600 BC in the Tian Shan Mountains of western China. There have also been further refinements in the study of the great plagues, as I will explain.

Plague lives most of the time in communal rodents, especially the kind that live in tunnels on open plains. It's oldest hosts seem to be marmots and ground squirrels of the Asian steppes. But after it crossed the Pacific to California during the Chinese plague outbreak of the 1890s it went native in North American prairie dogs, where it still lives, occasionally infecting Americans who spend too much time around wild prairie dogs, for whatever reason.

Citizens of Tournai bury plague victims, 1348

So why did the plague repeatedly break out of the marmot population and spread out of the steppes and into settled regions of China, Europe, and the Middle East? That is the big question. Plague can live for a while using rats or mice and humans as hosts, but this always seems to burn out after a time, with the disease eventually retreating back to the steppes. How does the disease jump, in a major way, from marmots to rats and people?

The two theories that have gotten the most play over the course of my lifetime have been the weather and geopolitics. 

Weather theories emphasize that the amount of land in central Asia wet enough for grain farming has changed over the centuries, expanding in wet eras and shrinking in dry eras. So if an area that had been too dry for farming, and was thus full of marmots, got wet enough to be plowed up, the disease-spreading fleas might jump from marmots to the mice and rats that live off grain, starting a cascading spread. This makes sense but not much has been done to refine the model, so far as I can tell because establishing the rainfall history of central Asia is extremely difficult.

The geopolitical theories focus on how likely it was that people, rats, and mice that had gotten the plague somewhere in central Asia would move into settled areas, bringing the plague with them. The big text here is Plagues and Peoples by William H. McNeill (1976) who argued that the two great plague episodes of western Europe happened because the whole steppes region was unified under great empires – the Huns in the 5th century, and the Mongols in the 13th – dramatically increasing the movement of people and their rodent friends across the region and into distant cities.

One problem with McNeill's theory is the timing; by 1346 when the Black Death got started, the Mongol Empire was already breaking up, so movement was more difficult than it had been back in the 1250s. But remember that most of the time, the plague is spread by flea bites, and maybe it just takes a while for it to become endemic in the rats and mice of a region and spread to the next region and so on. The theory fits quite well with the 19th-century Chinese plague, because in the second half of the 1800s the Chinese government was making a push to develop its desert areas in the west, for example by expanding irrigated farming, so there was probably a lot of movement between those areas and China's crowded cities.

One of the new articles getting attention is by British historian Peter Sarris; it was in Past & Present in 2021. Sarris showed that the genetic history of the several different strains of Yersinia pestis that had by then been identified in burials of the 6th century did not at all fit the narrative of Justinian's Plague that comes to us from Byzantine historians. The ancient narratives said that the plague appeared first in Egypt, spreading from there to Syria and thence to Constantinople. That would be very weird, since the plague has never been identified in wild African rodents, and McNeill dismissed that claim out of hand. The new evidence supports McNeill, since the most basal strain identified so far was found in burials in England. One plausible genetic tree has the plague beginning in central Europe in the mid 400s AD, that is, during the career of Attila the famous Hun. 

If confirmed, this is an important point: the disease seems to have entered the Roman world a full century before historians noted major outbreaks of plague. As to why it took so long to reach pandemic proportions, and why the pandemic then spread rapidly across areas that under this model may have recently been exposed to some level of plague, nobody knows. Anyway a model that has the plague starting in one city anywhere in the Mediterranean world and spreading from there appears to be dead.

The Mongol Siege of Baghdad

The other articles that have gotten a lot of attention are "The Four Black Deaths" by Miranda Green (2020) and "Plague and the Fall of Baghdad (1258)  by Green and Nahyan Fancy (2021). Fancy and Green argue that the Mongols brought the plague with them when they invaded Persia and the Middle East in the 1200s. They say they have Persian and Arab sources that describe the plague spreading with the Mongol armies, and that also describe shipments of grain brought into the Middle East from Central Asia. Persian historian al Shirazi wrote that after the Mongold captured Baghdad in 1258,

Emerging from their subterranean places of hiding, the few survivors saw Baghdad lying in complete ruin, its streets filled with the decaying and disfigured corpses of hundreds of thousands of the city’s former residents. . . all who survived the sword having died from the plague. 

Fancy and Green argue that the Mongol invasion led to Yersinia pestis getting established in the rats of Middle Eastern and Caucasian cities, and that

The fourteenth century plague outbreaks represent local spillover events out of the new plague reservoirs seeded by the military campaigns of the thirteenth century.

This is what the genetics of Yersinia pestis seem to be telling us: that both great plagues of western Europe somehow emerged from populations of the disease that had been established in the west a hundred years earlier. Thinking about the history of the Plague in Europe after 1350, this fits, since once the plague was established, Europe saw a bunch of local and regional plagues breaking out over and over for 350 years.

That still leaves me wondering what caused the two big plagues to break out in the first place, I mean, it takes a bit of gall to call the Black Death a "local spillover event." It must somehow be connected to the other unexplained thing about the plague, how it changes form. Most of the people who died in the Black Death were probably not bitten by fleas; apparently if enough people get the plague within a small area it begins to infect the lungs and becomes transmissable via airborn droplets, which we call Pneumonic Plague. If you get the plague this way, you get pneumonic plague, which is highly contagious, and the rapid spread gets going. This still happens around the world, often enough that the WHO is actively pursuing a vaccine. But how and why, we have no idea. And note that while pneumonic plague is far more contagious than bubonic, and epidemiologists think this is the only way to really have a plague pandemic, medieval writers on the plague all described lots of buboes (enormously swollen lymph nodes), which you only get with the flea-spread bubonic version.

One model of the big pandemics would be that the plague, long established as an endemic infection of rats and occasional infector of people, starts raging among the people and rats of a city and somehow makes the jump to the pneumonic form, which gets the real dying started, as well as the panicked flight that spreads the disease across the continent.

But that still leaves a whole lot to be explained.