Napoleon march to Russia in 1812: Typhus spread by lice was more powerful than Tchaikovsky’s cannonfire. - Slate Magazine
The common perception is that it was the combination of the extreme Russian climate (General Winter) as well as the brilliant tactics of Marshal Kutuzov (Borodino, burning of Moscow and the continued harassment of the French forces by the Cossacks). However a new book just published claims that it was typhus that was the ultimate undoing of Napoleon's Grand Armeé. Typhus is spread by the faeces of lice in unhygenic conditions. It was a big killer in the Great Famine three decades later in Ireland.