During the American War Between the States, about 1.6 Union soldiers died of disease for every soldier who died in actual battle. For the Confederacy, the figure was 2.5 dead from disease for every one killed in battle. This includes infection and complications from amputations and major internal surgeries done on tables out in the open. Germ theory and anesthesia were in their infancy then. For example, the surgeons working at the saw tables would use the same sponges from one patient to the next, simply squeezing it out in a bucket of red water in between patients. Some surgical kits came with a new substance called chloroform, but there often really wasn't time for that in the surgical assembly line. Medical assistants simply held the patient down for the surgeon. Even after thousands of amputations and surgeries, they said later**, one never fully got used to the agonizing jerks and shrieks of the wounded men.
I haven't studied other wars of the period, such as the war in the Crimea, but I imagine the statistics were similar with regard to dying of disease.
The U.S. is currently recognizing (you can't really use the word "celebrating") the 150th anniversary of our civil war.
What have we learned in the past 150 years? Well, we know about sanitation and sterility, and we have state-of-the-art anesthetics and pain killers. But...
**From "A Strange and Blighted Land" by Gregory A. Coco