Hard genetic evidence that lice-borne disease played a key role in Napoleon's disastrous retreat through Russia in 1812 has been produced by researchers.
Napoleon's 1812 Eastern campaign was a disaster
Experts analysed dental pulp extracted from the teeth of soldiers who died during the campaign.
They found lice-borne versions of typhus and trench fever ran rampant among the French Grand Army.
The study, by the Université de la Méditerranée in Marseille, is published by the Journal of Infectious Diseases.
Napoleon marched into Russia in the summer of 1812 with 500,000 soldiers.
Only a few thousand staggered out again, victims of war, weather, and disease.
Twenty-five thousand arrived in Vilnius that winter, but only 3,000 lived to continue the retreat. The dead were buried in mass graves.
Construction work in 2001 unearthed one such grave, containing between 2,000 and 3,000 corpses.
Researchers led by Dr Didier Raoult identified body segments of five lice in a forensic excavation of two kilograms of earth containing fragments of bone and remnants of clothing.
Three of the lice carried DNA from Bartonella quintana, which causes the disease commonly known as trench fever and afflicted many soldiers in World War I.
The team analyzed dental pulp from 72 teeth, taken from the remains of 35 soldiers.
Dental pulp from seven soldiers contained DNA from B. quintana, and pulp from three soldiers contained DNA from Rickettsia prowazakii, which causes epidemic typhus.
In all, 29% of the soldiers tested had evidence of either R. prowazkii or B. quintana infection.
The researchers said this suggests that louse-born diseases such as typhus and trench fever may have been a major factor contributing to Napoleon's retreat from Russia.
They believe analysing dental pulp for signs of DNA from infectious agents may become an important tool for investigating the history of communicable diseases.
Dr Carole Reeves, a medical historian, said it was ironic that teeth were now uncovering the health secrets of Napoleon's army, as they were widely salvaged at the Battle of Waterloo in order to make dentures.
"Wherever there is warfare there is always infectious disease," she said.
"And up to World War I many deaths were caused by infectious disease rather than the warfare itself."