Start with the number, because it is difficult to hold in the mind.
The older textbook figure was a third. Detailed work on the surviving records over the last forty years has pushed it up. For many well documented regions the range now argued is somewhere between forty and sixty per cent of the population dying within about two years of the disease arriving, with some communities losing more and a few escaping almost entirely.
Try to scale that to anything in modern experience and it does not work. The worst year of the influenza pandemic of 1918 killed on the order of two to three per cent of the world's population. Take the most destructive war in European history, the one that is chapter forty seven, and its total death toll across six years is a fraction of what one epidemic did in two.
It also did not go away. It came back roughly every ten to twenty years for three centuries, in outbreaks that were smaller but which prevented recovery, so that the population of Europe did not regain its 1300 level until somewhere around 1500 or later.
For a long time the identification was an inference from the symptoms described in the sources, and it was seriously disputed. That part is now settled.
The bacterium Yersinia pestis has been recovered from the teeth of people buried in fourteenth century plague cemeteries, in London and at sites across Europe, and its genome has been sequenced. The organism is real, it was there, and the strains involved are ancestral to the ones circulating today.
What is not settled is how it spread, and the argument is live and interesting. The standard account involves a bacterium living in rodent populations, transmitted by fleas, reaching humans when the rats die and the fleas look for another host. That mechanism is well established for modern outbreaks.
The objection is that the fourteenth century epidemic moved much faster than rat borne plague normally does, spread rapidly in winter and in northern climates where the relevant fleas do poorly, and did not leave the trail of dead rats that observers of modern outbreaks always report. Proposed alternatives include transmission by human fleas and body lice, which would make it a disease of human contact rather than of rodents, and a larger role for the pneumonic form, which passes directly between people through the air and is almost always fatal. Modelling studies have found the human parasite route fits the mortality patterns better in several cities. This is not a fringe position, and anyone who tells you the rats are simply the answer is a few decades out of date.
It came from the east along the trade routes, and it moved at the speed of commerce.
The traditional origin story has it reaching a Genoese trading post in the Crimea during a siege in 1346, where the attacking army is said to have catapulted its own dead over the walls. That detail comes from a single account written by a notary in Italy who was not present, and it should be held loosely; what is not in doubt is that ships carried it west from the Black Sea.
It reached Sicily in October 1347, Genoa and Marseille within weeks, and then moved along the coasts and up the rivers. Paris and southern England in 1348, the rest of England, the Low Countries and Germany in 1349, Scandinavia and the Baltic in 1350, Russia by 1353.
The map of its spread is essentially the map of the trade network built in chapter sixteen. Ports first, then the towns on navigable rivers, then inland. Some isolated regions were passed over almost entirely, and those exceptions, in parts of Poland, Bohemia, the Pyrenees and Iceland at this stage, are among the better arguments that transmission depended on human movement.
The medical response was intelligent within a false theory. Physicians worked from the accepted account of disease as corrupted air arising from a bad conjunction of planets, and so recommended flight, avoidance of low damp places, aromatic substances to counteract the bad air, and the avoidance of hot baths, which opened the pores. Flight worked. The rest did not.
The administrative response was more consequential, and it was Italian. Cities appointed boards of health with emergency powers, closed gates, restricted movement of goods, and isolated the sick. In 1377 the port of Ragusa on the Adriatic ordered arrivals from infected places to spend thirty days on an island before entering. Venice extended the period to forty, and the Italian word for forty is where the word quarantine comes from.
That is a genuinely important moment. It is the first appearance in Europe of a permanent public health administration, with statutory powers to override property and movement in the name of the population's survival, and it was invented by city governments improvising.
The religious response ran in two directions. One was intensified devotion, processions, and the flagellant movement, in which bands of men marched between towns whipping themselves in public penance. They were popular, they were uncontrollable, they spread the disease efficiently, and the pope condemned them in 1349.
The other was to find someone to blame. From 1348 accusations spread that Jews had poisoned the wells, extracted under torture and circulated in writing between towns. Jewish communities across the Rhineland, Provence, Aragon and Germany were destroyed, often before the plague reached the town. In Strasbourg in February 1349 several hundred to two thousand Jews were burned, with the plague not yet arrived and the town council that had opposed the killing overthrown to permit it. The pope issued bulls stating that the accusation was false and that Jews were dying of the disease like everybody else. It made no difference.
The clinical picture in the sources is consistent enough to be worth stating, because the abstraction of a percentage hides what people were actually watching.
The commonest form began with fever and a swelling in the groin, armpit or neck, a hard painful lump the size of an egg or larger, from the Greek word for which we get the name bubonic. Black or livid blotches spread on the skin. Most who reached that stage died within about a week. A form that attacked the lungs killed faster, in two or three days, and those patients coughed blood and infected everyone near them.
What the chroniclers dwell on is not the medicine but the collapse of the rituals. Funerals stopped. Bodies were left at doors for carts. Grave pits were dug and filled in layers, and excavations in London have found exactly that, rows of bodies laid out carefully in trenches, which incidentally shows that the burial teams kept working with some order under conditions that the written sources describe as chaos.
One Italian writer, whose father was a merchant and who lived through it in Florence, recorded that brother abandoned brother and parents refused to nurse their own children, and that this was the thing people found hardest to forgive afterwards. Another, a Welsh poet, wrote a lament for his children in which the swelling is described as a head of an onion, ashes, a burnt cinder. That is not statistics. It is the closest thing to a voice the period has left.
And the clergy died disproportionately, because their duties took them to the dying. Records of appointments to parishes let us count the replacements, and in some English dioceses the turnover in 1349 runs to nearly half the parishes in a single year. That is how we know the mortality rate as precisely as we do, and it is also why the church came out of the epidemic with a shortage of trained priests that took generations to fix.
Now the part that is genuinely counterintuitive. For those who lived, the long term economic consequence was an improvement, and a large one.
The plague killed people. It did not destroy land, buildings, tools, animals or stored wealth. So the survivors inherited an economy with the same capital and a much smaller workforce, and the price of labour is set by its scarcity.
Wages rose sharply. In England, where the wage series is best, real wages for building craftsmen roughly doubled over the following decades and stayed high for a century and a half, a level not reached again until the nineteenth century. Rents fell. Landlords could not find tenants for holdings that had been fought over a generation before, and had to offer better terms or leave the land empty.
The response of the people losing out was legislation. England passed an ordinance in 1349 and a statute in 1351 attempting to freeze wages at pre plague levels and to compel the able bodied to work for anyone who offered those rates. Similar measures were passed across Europe. They were enforced with real energy, they generated a great deal of litigation, and they largely failed, because an employer who wanted the work done had an incentive to break the law quietly.
The political consequence arrived a generation later. The English rising of 1381, triggered by a poll tax, was carried by people who had lived through both the wage controls and the attempt to reimpose old labour services, and its demands included the abolition of serfdom outright. It was crushed and its leaders killed, and serfdom in England dwindled away over the following century regardless, because the economics were against it.
The same shock produced the opposite result in eastern Europe. There, lords responded to labour scarcity by using their political dominance to tie peasants to the land more tightly, and serfdom hardened rather than dissolved. The same disease, the same labour shortage, and two opposite outcomes, which is a useful warning against explaining history by economics alone.
Firm: the pathogen. Ancient DNA has settled what it was, and this is one of the clearest cases of a scientific technique resolving a long historical dispute.
Firm: the economic aftermath, at least where wage and price series survive. The English data in particular is good enough to see the change happening decade by decade.
Soft: mortality. Everything rests on manorial records, tax lists, wills and clerical appointments, each of which samples a particular slice of the population. The figures in the first section are honest ranges, not measurements, and they differ by region by more than the average conceals.
Contested: transmission, as above. Genuinely open.
Contested: the wider cultural claims. It is often said that the plague produced the Renaissance, the Reformation, individualism, or a new realism about death. These arguments are attractive and mostly untestable, and it is worth noticing that they tend to be made about whichever development the writer already wanted to explain. The economic effects can be measured. The spiritual ones are inference.
A last thing worth holding onto. Contemporaries wrote that the survivors were changed, that people were more grasping, less charitable, quicker to litigate, and that the old bonds had loosened. That is what people always say after a catastrophe, and it may be projection. It may also be what a society looks like when nearly everyone in it has buried half the people they knew.