*** “Add in malnutrition and lack of medical care, and you can take your pick as to what finally carried them off” ***
No Penicillin ?
The foundations of modern medicine are relatively recent. Anesthesia was being mainstreamed in the 1850's; Queen Victoria was a trailblazer for delivering a child while anesthetized, and the Civil War was the first major war fought with the general availability of anesthesia, at least once the wounded got to a field hospital. It therefore marks a dramatic point of acceleration for surgery, as surgeons were now able to take more time and go deeper, as opposed to quick amputations, splinting and bandaging.
The germ theory of disease was just over the horizon. Some of the seminal research was already being done, but Robert Lister would not publish his seminal paper until a few years after the war. The germ theory of disease led to the practice of antisepsis, which was woefully lacking prior to that point. Antisepsis and sterilization would have saved countless lives during the Civil War, but that's 20/20 hindsight.
Antibiotics began to be developed only in the 1920's. The results are still within living memory. I had an uncle (who I of course never met), for example, who died at age 12; a shot of penicillin would have saved him, but it didn't exist.
Blood transfusions come along still later; I'm not going to look it up now. I'm sure someone will be along with the date.
Medicine was still in the dark ages 160 years ago.