Plague didn't need crowded cities or flea-infested rats to be deadly. A landmark study published in Nature by researchers from the University of Copenhagen and Cambridge reveals that Yersinia pestis — the bacterium behind plague — was already killing prehistoric hunter-gatherers in Siberia around 5,500 years ago, making these the oldest known plague outbreaks ever documented. The team analyzed ancient DNA from 46 individuals buried at four cemeteries near Lake Baikal, finding plague evidence in 18 of them — a detection rate rivaling some medieval plague pits.

What's striking is how deadly these early strains were. Though they lacked the flea-transmission genes of later bubonic plague, they carried a superantigen — a toxin-producing genetic factor that can trigger extreme immune responses — not found in any later plague strains. This likely explains why children and young teens were disproportionately among the dead, with siblings and family members buried together after what appear to be rapid, community-wide outbreaks. Marmots, which still carry plague today, are suspected as the original source.

By the Numbers

• 18 of 46 individuals (~39%) tested positive for Y. pestis — higher than some medieval plague burial sites
• Two distinct outbreak phases identified, separated by ~4–6 centuries
• Oldest confirmed plague outbreak: ~5,500 years ago; next oldest is ~5,000–5,300 years ago in Latvia
• Mortality peaked in children aged ~7.5–11 years

Why it matters: These findings overturn the long-held assumption that large, lethal plague outbreaks required dense agricultural populations. Even small, mobile communities were vulnerable — and the pathogen was already highly dangerous at its earliest known stage.