People who they thought had little chance of living were left for last. Men with abdominal injury's usually bled to death since they weren't cared for soon enough. People who needed any limbs cut off were usually given morphine or chloroform to reduce the pain. Opium Pills were also dusted into the wound. However, doctors knew nothing about addiction and soon many soldier became addicted on these pills.
Battle wounds weren' t the only cause of infection though.
And due to the shortage of water, surgeons often went for days without washing their hands or equipment. They rarely ever got a chance to wash their clothing, which may have been blood and pus stained. And the operating rooms were very poorly taken care for. There were many other factors of death as well. Many younger men suffered the Mumps, Measles, Chicken Pox, or a terrible cough.