This paper will discuss the effects of infectious disease on Napoleon's forces during the Russian campaign of 1812. In short, it will argue that the primary reason Napoleon failed to defeat the Russian army was because his forces were decimated by disease, specifically typhus, dysentery, and diphtheria. It will also demonstrate the effect of disease and illness on Napoleon's judgement and decision making process. This subject, infectious disease and the military, has great implications for military planners in the future. The recent Gulf war and its related "Gulf War Illness" is just one example. The United States has lost more men to disease during war than any other cause; hence it is critical that today's and tomorrow's military leaders are aware of the dangers. The scope of this paper is bounded by the Russian campaign timeframe, but it will concentrate on the march to Moscow; specifically the events that occurred upon entering Polish and Russian territory. Also, there were two major battles fought prior to reaching the gates of Moscow which this paper will show were the two decisive points in the campaign. These battles occurred at Smolensk and Borodino, and they illustrate key instances where Napoleon's leadership, judgment and decision making come into question, not only by historians and authors, but by several of Napoleon's own generals. The Russians were outnumbered in the summer of 1812 and were forced to play at fight and retreat game with Napoleon's army. This was not well received by the Russian people who were suffering at the hands of the French invaders, but it was just about the only sound course of action the Russians could pursue. The situation turned itself around however when Napoleon reached Moscow and stayed there until an orderly and safe retreat was nearly impossible. At this time the Russian winter was upon them and they had neither the will nor the supplies to return to France.