How old is the plague?

A 2015 study screened the teeth of over a hundred Bronze Age individuals from Eurasia and found Yersinia pestis in seven of them, the oldest from graves dated to about 2800 BC.

That is three thousand years before the first historically recorded plague pandemic. The bacterium was infecting people across Eurasia during the period of the great steppe migrations.

Teeth are used because the pulp inside them is sterile and durable, a lasting refuge for a bacterium that was in the blood when its host died.

The early strains lack a gene that later strains have. It makes a protein that protects the bacterium inside a flea’s gut, which is what lets a flea carry it from one animal to the next. Without it, flea transmission does not work.

So these ancient infections spread another way, probably by breathing or by direct contact, and caused a different kind of disease. The gene appears in strains from about a thousand BC onward.

A sequence of genetic changes, dated, that turned one disease into another.

In 2011 a draft genome was recovered from the teeth of victims buried in a London plague cemetery between 1348 and 1350, which identified the organism behind the Black Death directly rather than from chronicles.

The strain turned out to sit close to the ancestor of most plague lineages circulating today, and it carries no distinctive mutation that would explain the severity of that pandemic.

So the virulence of the Black Death is probably not explained by the pathogen being different. Whatever made it what it was involved the people, the climate, the fleas and the conditions they all lived in, and that is still being worked on.

The Bronze Age strains coincide with large population movements across the steppe and with turnover in the genetic ancestry of Europe. The 2015 authors call it plausible that outbreaks helped drive those movements, or were driven by them. That is a coincidence in time, not a demonstration.

The team that sequenced a five-thousand-year-old strain from a hunter-gatherer burial in Latvia reads it the other way: the few prehistoric cases found so far look to them like separate jumps from animals rather than an epidemic.

Seven positive individuals is a small sample from a vast region, and prevalence cannot be estimated from it.

The same methods have settled arguments that descriptions could not. Whether the plague of Justinian was plague at all was disputed for decades, until the bacterium was recovered from teeth in a sixth-century Bavarian cemetery.

The limit is preservation. Diseases that do not enter the bloodstream, or that leave no DNA in bone, remain invisible, so the record that is being built is biased towards a particular kind of infection.