Medicine in the American Civil War
One problem was that, like in World War I, people had just invented new weapons, like the Minié ball.  Guns with rifled barrels did not miss as often as the muskets from the Revolutionary War and other earlier wars did.  That meant doctors did not already know how to treat the injuries that these weapons could cause in the human body.  Another problem was that the camps and battlefields were very dirty.  A soldier in the American Civil war was twice as likely to die of disease as of a battle injury.