Final answer:
Typhus was the contagious disease that killed many French troops in Russia.
Step-by-step explanation:
The contagious disease that killed many French troops in Russia was typhus. Typhus is a bacterial infection transmitted by lice that causes a high fever, headache, rash, and muscle pain. It can be fatal if left untreated. The contagious disease that inflicted significant casualties on French troops during Napoleon's ill-fated invasion of Russia in 1812 was typhus (a). Typhus, a highly contagious bacterial infection spread by lice, flourished in the unsanitary and crowded conditions of the military campaigns. The Grand Army, facing brutal winter conditions and strained supply lines, suffered not only from the harsh climate but also from the devastating impact of diseases like typhus. The spread of typhus contributed to the immense losses suffered by Napoleon's forces during their retreat from Russia, playing a crucial role in the military disaster that unfolded during the campaign.