Disease is pretty bad; Almost everywhere during the Napoleonic Wars there was some epidemic spreading through the army.
In the West Indies, Yellow Fever was a major roadblock for the British and French armies, and the civilian population also.
Yellow Fever is carried by the nasty bug, the mosquito (don't they cause everything?). Yellow Fever killed so many that
every soldier stood a 1 in 2 chance of dying from it if he was posted in the West Indies.
Egypt was pretty bad too, there was heat, lack of water and shade, and contaminated water. When the French army
made it to the Holy Land, they were struck down by Plague.
Ophthalmia, caused by Chlamydia Trachoatis was a common cantagion in the British Armies. It caused scarring of
the cornea and blindness. The French and the British used different ways of stoping the infection.
Larrey again came to the rescue. There were no cases of blindness ever reported in the French Army. In the
British army, that nine years later, there were still some 2,500 men suffering from trachoma and blindness in the army.
In Russia the freezing cold was pretty nasty for the retreating french and they were plaqued by various diseases.
As always, disease was the leading killer in the armies and in the civilian populations of that time.