Scientists recently discovered deadly bacteria in DNA from Napoleon's soldiers' teeth, revealing new causes of death during the catastrophic 1812 Russian retreat.

Scientists say they've discovered traces of the deadly pathogens that ravaged Napoleon's soldiers during his doomed 1812 retreat from Russia — offering a clearer picture of the circumstances of the French general's downfall.

A study published in the journal Current Biology on Nov. 3 analyzed DNA from the teeth of French soldiers who were buried in a mass grave in Vilnius, Lithuania.

The soldiers served under Napoleon during his disastrous invasion of Russia in the winter of 1812. They were among the roughly 300,000 French Grande Armée soldiers who perished in the campaign.

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The teeth contained bacteria that causes paratyphoid fever and louse-borne relapsing fever, said the researchers — indicating that several infections circulated among the soldiers who were weakened by hunger and the freezing temperatures.

These two pathogens had not been identified previously among Napoleon's soldiers, though historians have documented the harsh conditions of their deaths for over two centuries.

After marching into Moscow, Napoleon's campaign quickly unraveled. The French emperor was forced to retreat amid the harsh Russian winter, dwindling supplies and fierce counterattacks.

Study author Nicolás Rascovan, speaking to Reuters, said Vilnius "was a key waypoint on the 1812 retreat route." The mass grave contains the remains of up to 3,000 soldiers.

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"Many soldiers arrived exhausted, starving and ill," the microbiologist said. 

"A substantial number died there and were interred rapidly in mass graves."

Rascovan said that starvation, numbing temperatures and typhus were long known as major causes of death, but that the study conclusively proves that paratyphoid fever and louse-borne relapsing fever were also present.

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These "may have contributed to debilitation and mortality," Rascovan added.

Four of the 13 soldiers tested positive for the paratyphoid fever bacterium, while two others tested positive for the relapsing fever bacterium. 

Both bacterial infections cause fever, headache and weakness, and can spread quickly in crowded, unsanitary conditions.

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Paratyphoid fever is spread through food and water, while louse-borne relapsing fever is spread by body lice.

Rascovan emphasized that studying ancient DNA "lets us put names to infections that symptom-based accounts alone cannot resolve."

"The co-occurrence of pathogens with different transmission routes underscores how dire sanitary conditions were," he said. 

"Future work across more sites and individuals will refine the disease landscape of 1812."

Reuters contributed reporting.