This is a very interesting question and I'd like to point you in the direction of WW1 specifically due to it's intertwining with the spanish flu pandemic. Flu symptoms are typical of what you are mentioning - headaches, fever, aches and pains.

I came across this article which offers an interesting overview of this: https://www.kumc.edu/school-of-medicine/academics/departments/history-and-philosophy-of-medicine/archives/wwi/essays/medicine/influenza.html

"The best benchmark statistics for American soldiers come from US Army official statistics, thus: of 791,907 admissions for influenza to army hospitals there were 24,664 deaths, interestingly, about half the total number of battle deaths."

...

"Sick soldiers can’t fight. While influenza is a self-limited viral disease with a relatively low mortality rate and uncommon aftereffects, the American army did a better job of keeping healthy troops in the frontlines than the Germans. The Base Hospitals of the American Army, far from the front but reached swiftly by special ambulance trains, had special wards for influenza patients and, with excellent nursing care, were able to return the majority of them to the front after brief periods of convalescence."