Excluding extreme cold, tropics, and common epidemics of the time, why would disease be such a big problem? Did commanders do calculations such as "Yeah I have 10 thousand men right now, but when we get to x city , only 6 thousand will remain, so I should plan accordingly"? And which disease would be most likely to kill a soldier in summer in Central Europe, let's say in Seven Years War?
When Henry V attacked Harfleur in 1415 he lost very few to combat, but upwards of 1500 were sent home sick with dysentery. Attacking Calais, his grandfather Edward required repeated reinforcement to cover losses due to disease. Id.
Dysentery is a bad-water illness, very debilitating, certainly can be fatal, and is a natural consequence of premodern bivouacking: men and horses all crapping together with no regard for sanitation. It affected pretty much every army on campaign, and was characterized here as "more fatal than powder and shot." Retrospective diagnosis of overall disease casualties from 200 or 600 years ago isn't really possible, but I would nominate dysentery as the most likely cause of non-combat casualties. As the author points out, of 12,535 war deaths in the US Mexican campaign, 10,986 were due to disease from camp water pollution, presumably dysentery.
Henry did, in fact, alter his strategy after his losses at Harfleur, leading his depleted force north in semi-retreat, toward the shelter of the English beachhead at Calais. He ran into the main French force at a field known as Agincourt, but that's another story. .