Shell Shock

Combat stress reaction is another way of saying Shell Shock or PTDS, which is post traumatic demoralization syndrome. In WWI a big amount of the soldiers had this, it was the aftermath of what they'd experience. Life was extremely difficult for them, sleeping, walking, talking were things they struggled on. It's incredibly hard for them to over come Shell Shock, some do over come it, while the others still suffer from war and the horrible moments they went through.
Before doctors started to find out what it was all about they'd just blame it on other things that could of caused your behavior. For example they thought that the lack of moral fiber was a thing that could trigger shell shock. Followed by inhaling dirty air, or getting attacked and being close to death. Soldiers out there were so close to death every time they stepped out in the battlefield. Later on they found out the real reason on why these people were experiencing this.
It had become a pressing medical and military problem, it didn't only affect the front line troops and soldiers but also the doctors. They were also having difficulties with treating this disorder, they didn't know what was happening and how to treat it. Charles S. Meyers came to the rescue by being a medically trained psychologist. He gave his opinions and collected data to see deeper into it and know what he was dealing with.
Shell shock doesn't only occur during war but after when you go back home. There are days when you wake up thinking that you're about to fight, when really you're just waking up from a dream. Many people experience this horrific thing. Waking up thinking you're somewhere else, having your family looking at you with strange faces. Moreover not many people actually fully recover, it comes back at the most unappreciable times.
http://www.bbc.co.uk/history/worldwars/wwone/shellshock_01.shtml