Your idea of the Middle Ages stretches over a thousand years, give or take. Can you specify? A lot changes with regards to medicine and combat within a millennium.

I think it can be safely assumed that a large number of combatants in any medium length campaign would be sick - this was true up until WWII really. There are countless primary sources which attest to soldiers with dysentary shitting themselves through their armor, or collapsing with illness, or vomiting through their helmets, etc. Illness and war have gone together since war first started and soldiers would very often have to fight even with debilitating illness. How much was always a question of medicine (both technology and available stores at present) and the necessary number of soldiers for any given battle.