The plague knows how to take it easy

The history of the plague is primarily remembered for its devastating nature, sweeping away human societies struck by the scourge in a matter of months. The first waves of the three historical plague pandemics—the Justinian plague from 541 to 544, the Black Death from 1346 to 1351, and the Oceanic Plague from 1855—hit like tsunamis: it is estimated, for example, that the Black Death wiped out 25 million people in Europe in five years.

And then? None of them suddenly died out. The Plague of Justinian lasted well after his death, until 750; the great medieval plague resurfaced until 1840 with regular outbreaks in Europe and around the Mediterranean basin; finally, the third pandemic never ended: now endemic in different regions of the globe, it reemerged sporadically, passing from wild rodents to urban rats and humans.

Yet none of the subsequent aftershocks have been as widespread as the first waves. The disease remains contagious and often fatal, but the spread is less rampant, and the number of victims is reduced. Several factors may be responsible for this relative calming: a reduction in rodent populations, decimated during the main wave; a selection in the human population of protective variants among the genes involved in the immune response; a change in behavior; or even a reduction in the virulence of the bacterium responsible for the plague, Yersinia pestis .

An article published on May 29 in the journal Science , the result of a collaboration between several research teams from McMaster University in Ontario, Canada, and the Pasteur Institute, highlights a modality of virulence attenuation common to all three pandemics. By comparing Y. pestis genome sequencing data from ancient DNA samples and recent isolates, they found that some strains, representative of late resurgences in the three pandemics (these strains appeared around the years 650, 1420, and 1960, respectively), have partially lost the same portion of DNA in their genome.

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