One of civilizations most prolific killers shadowed humans for thousands of years without their knowledge.
Scientists long assumed that the deadly disease began infecting humans just before the earliest epidemic, the Justinianic Plague.
Why didnt these earlier infections lead to devastating outbreaks like the Black Death?
If fossil teeth containY.
pestisDNA, its safe to assume that person died from plague.
Several studieshave foundplague victimswho lived nearly 5,000 years ago more than three millennia before the first known plague epidemic.
Pathogen aDNA analysis also revealed howY.
pestisbacteria have evolved over time.
For example, laterY.
pestissamples lack the gene.
The mutations were also identified in a skeleton from Iron Age Armenia that was dated toaround 950 BC.
Its possible outbreaks occurred but the evidence simply hasnt been found yet.
A mass grave of plague victims from the early 18th C in Martigues, France.S.
Alternatively, there could be another explanation, related to the behavior of the people being infected.
First we established conditions that make a population more or less vulnerable to an outbreak.
We identified criteria known to be associated with plague virulence, or how infectious the bacterium is.
These rodents make ideal hosts for fleas that harbor plague bacteria.
As East Asia is thelikely geographic source of plague,regular tradewith the region is another factor.
Regular contact with horses could reduce a populations susceptibility to the disease.
Constantinoples culture created a perfect storm of conditions for an epidemic.
It was a congested urban center with a population of over 500,000 people, or140 individuals per acre.
In contrast, lifestyles in Samara and Armenia may have kept the epidemic at bay.
These populations were significantly more mobile and less congested than the urban population of Constantinople.
The Samara population showslittle evidence for agricultureand tended to occupy small settlements of extended families.
These communities managed shared herds, and horse tools found in their characteristicburial moundssuggest the animals were highly valued.
Remember, horses may have had some natural immunity to the disease.
Less congestion would have made contaminating nearby villages more difficult.
Lacking agriculture, Samara could not have supported human-dependent rodents, the way Constantinople did.
Both populations potentially benefited from a high ratio of horses to people.
Vulnerability to plague was an unintended consequence of this societys lifestyle.
Meanwhile, it seems earlier cultures unwittingly shielded themselves from the same threat.
But understanding how human behaviors affect the spread and virulence of a disease can inform preparations for the future.
Human behaviors are just as critical to our disease susceptibility as are the characteristics of the pathogen itself.
