In the summer of 1812, Napoleon Bonaparte led half a million soldiers into Russia. By the end of the year, only a fraction survived.
Historians have long blamed typhus, a deadly louse-borne disease, for the disaster. But new research suggests otherwise.
Scientists from the Institut Pasteur in France used advanced DNA analysis to reexamine the remains of Napoleon's fallen troops.
They found no evidence of typhus. Instead, the soldiers carried bacteria that cause enteric fever and relapsing fever, two diseases that may have hastened the army's downfall.
"It's very exciting to use a technology we have today to detect and diagnose something that was buried for 200 years," says lead author Nicolás Rascovan of the Institut Pasteur.
For over two centuries, researchers have debated what wiped out Napoleon's Grand Army during the failed invasion of Russia. Contemporary doctors and officers reported symptoms consistent with typhus.
The discovery of body lice and traces of Rickettsia prowazekii, the bacterium behind typhus, on remains found in past excavations further reinforced that belief.
However, the new study challenges this long-held assumption.
Rascovan's team revisited samples taken from a mass grave in Vilnius, Lithuania, located along the French army's retreat route.
Using advanced microbial paleogenomics, they extracted DNA from the teeth of 13 soldiers.
Teeth, which preserve biomolecules well, provided the most reliable source of ancient DNA.
After removing environmental contamination, the team sequenced and analyzed the genetic material.
What they found surprised them: there were no traces of R. prowazekii or Bartonella quintana, the bacterium behind trench fever.
Instead, two other pathogens emerged, Salmonella enterica and Borrelia recurrentis.
The first causes enteric fever, a group of illnesses that includes typhoid, while the second triggers relapsing fever, a louse-borne disease characterized by recurrent high fevers.
The team's broader sequencing approach not only identified the bacteria but also revealed a surprising evolutionary link.
The B. recurrentis strain found in Napoleon's soldiers matched a lineage discovered in Iron Age Britain, dating back 2,000 years.
That ancient lineage somehow persisted in Europe for millennia before eventually being replaced by modern variants.
"This shows the power of ancient DNA technology to uncover the history of infectious diseases that we wouldn't be able to reconstruct with modern samples," says Rascovan.
The findings offer a new perspective on one of history's most famous military disasters.
Instead of a single cause, multiple infectious diseases combined with hunger, exhaustion, and freezing conditions likely decimated Napoleon's army as it retreated from Russia.
By applying cutting-edge DNA analysis to a 200-year-old mystery, researchers are shedding new light on what truly killed thousands of soldiers during one of the most brutal campaigns in history.