Ever since his catastrophic retreat from Moscow, the terrible losses suffered by Napoleon's soldiers have been blamed on hunger and the biting cold of the Russian winter. But according to new research by French scientists, the fabled Grande Armée, reduced to 30,000 men by December 1812 from a total of 600,000-700,000 just six months earlier, was actually felled by parasites.

Researchers led by Didier Raoult of the National Scientific Research Centre in Marseille have analysed the DNA of 72 teeth extracted from 35 skeletons removed from a mass military grave near Vilnius, in present-day Lithuania, in 1995.

They found minute traces of the microbes associated with typhus and trench fever, deadly diseases transmitted by fleas and lice, in the dental matter of 10 Napoleonic soldiers, the newspaper Le Figaro reported yesterday.

The scientists confirmed the typhus hypothesis by identifying the remains of three fleas in 5kg of soil and human remnants taken from the grave, in the grounds of a former army camp that was occupied in turn by Tsarist forces, Hitler's invading troops and finally the Soviet army.

"We are confident in this diagnosis," one of the team, Pierre-Edouard Fournier, told the paper. "Cold and hunger certainly claimed many victims, but diseases greatly increased the toll."

The Grande Armée, at 691,000 men the largest fighting force assembled in Europe, crossed the river Neman in Lithuania on June 23 1812, and began its long march towards Moscow.

Meeting only sporadic resistance, Napoleon reached his objective in early September. The invaders found Moscow empty of people and supplies, and much of the city immediately went up in flames in what is assumed to be sabotage by Russian fighters.

The Corsican-born emperor abandoned the ruined city - without ever receiving the formal surrender of the Russian army - on October 19. It immediately became clear that his retreat was going to be one of the most celebrated military debacles in history.

Constantly harrassed by Russian forces, including the fearsome Cossacks, and forced to take the same scorched road west on which it had earlier moved east, the French army soon lost almost all its horses and had to continue the harrowing withdrawal on foot.

Until now, it has been assumed that the Russian winter in effect finished the job, killing hundreds of thousands through frostbite, hunger and exhaustion. But according to the scientists, the conditions of the retreat favoured a mass infestation by lice and fleas.