In October 1812, when Napoleon commanded his troops to retreat from Russia, a catastrophic series of events unfolded. Battling starvation, freezing temperatures, exhaustion, and illness, approximately 300,000 soldiers perished during the withdrawal.

Recent research has uncovered surprising evidence of two diseases—paratyphoid fever and relapsing fever—afflicting the soldiers, offering new insights into the challenges they faced during this historic retreat.

“The critical factors behind the catastrophic failure of the retreat were indeed the cold, hunger, and other hardships. Without infectious diseases, the death toll would still have been significant,” explained Nicolás Rascovan, head of the microbial paleogenomics unit at the Institut Pasteur and a contributor to the study. “However, this discovery enhances our understanding of the array of infectious diseases involved.”

In their publication in the journal Current Biology, Rascovan and his team detail how prior DNA analyses from soldiers interred in a mass grave in Vilnius, Lithuania, had already indicated the presence of typhus and trench fever.

These earlier findings were derived using a highly sensitive method known as nested PCR, which involves detecting specific pathogens in samples.

Rascovan’s team employed a different approach called shotgun sequencing, which enabled them to identify DNA fragments corresponding to any of 185 bacteria known to cause human diseases.

The results, based on DNA from the teeth of 13 soldiers who had not previously been studied, revealed one soldier had been infected with the louse-borne bacterium Borrelia recurrentis, which causes relapsing fever, and four others had been infected with a type of the bacterium Salmonella enterica, which causes paratyphoid fever, a disease spread by contaminated food or water. One of these four soldiers might also have had relapsing fever, the team said.

The researchers say the findings fit with historical descriptions of the symptoms experienced by soldiers of Napoleon’s Grand Armée, such as fever and diarrhoea.

Unlike the previous studies, however, the team found no traces of the bacteria that cause typhus or trench fever. While Rascovan noted it could be that these soldiers were not infected with those diseases, or had only a mild infection, the results could alternatively be explained by the breakdown of ancient DNA over time, or that the quantity of DNA present was below the detection limit of the technique used.

The researchers carried out a number of statistical tests and analyses to ensure their results were robust and pointed to genuine infections. These included looking for the signs of DNA degradation that would be expected from authentic ancient DNA, and exploring where the DNA belonged on the evolutionary “family tree” of the two bacteria.

“In light of our results, a reasonable scenario for the deaths of these soldiers would be a combination of fatigue, cold, and several diseases, including paratyphoid fever and louse-borne relapsing fever. While not necessarily fatal, the louse-borne relapsing fever could significantly weaken an already exhausted individual,” they write.

Dr Michael Rowe, an expert in European history at King’s College London, welcomed the study.

“The science is interesting because I think it does something which a historian couldn’t do,” he said, referring to the identification of the diseases.

But he cautioned against assuming the devastation of the army was simply down to harsh weather that left soldiers susceptible to starvation and disease – a view Napoleon promoted.

“That underplays the Russians and it underplays the fact that they actually do some very clever things and that they [have] got a very good strategy and they’ve got actually quite a sophisticated army,” he said.