Though battlefield medicine has been documented and practiced since before Upper and Lower Egypt were united, the United States Civil War was the first time that the concept of treating wounds during a battle (and not killing medics!) was really pervasive. That was the first time that there were multiple dedicated medics (even if not well-trained) per unit, and at least one ambulance cart per regiment. Union military physicians Joseph Barnes and Jonathan Letterman worked to ensure this, as well as worked to design an effective prehospital care system.
Casualties were still huge, and the dangers of infection were there both on the field and if one had surgery performed. Something that most people don’t realize about Civil War medicine is that when available (basically anywhere besides the quagmires of the Deep South and when there were in insurmountable number of injured who needed surgery), around 90% of surgeries done on Union soldiers were done using a general anesthetic- chloroform. Since most of the surgeries were relatively quick (amputations and bullet removals), there were actually relatively few deaths caused by anesthesia problems.