When a survivor is shot and he/she manages to live, there will be no physical disadvantage to the character after bandaging, healing blood/health back to full, and dropping the pain and shock and bone status to normal with a morphine.

But bullets should leave shrapnel within the body. This should effectively stop a survivor from healing properly. Surgical operations should be implemented to remove these shrapnel.

Without removal of shrapnel, there should be disadvantage to a survivor who doesn't take care of his/her body; such as limping, swaying of aim, constant increase of pain and shock etc.

Surgery should involve blood loss and increase in pain and shock values. This feature will bring back the need of saline bags, blood bags and painkillers. Nobody uses these because they aren't technically essential. Surgery will prevent blatant 'natural healing' in case where medication is clearly required.