Soldiers returning from World War 1 suffered from a wide range of conditions collectively described as shell shock, which resulted in symptoms ranging from panic attacks to neurotic paralysis of the body. According to HealthGuidance, these types of mental effects are now formally referred to as post-traumatic stress disorder (PTSD).
WebMD states that PTSD is "a serious condition that can develop after a person has experienced or witnessed a traumatic or terrifying event in which serious physical harm occurred or was threatened." The cause of shell shock in World War 1 soldiers was due to the traumatic scenes they had witnessed combined with a large amount of time spent away from civilization. Soldiers were often under constant machine gun fire and witnessed fellow soldiers killed by enemy bombs and ammunition.
Former soldiers have been known to react strongly to anything that reminds them of the traumas they experienced and may begin to avoid anything they associate with them. Soldiers may also be reluctant to mingle socially due to loud noises that remind them of bombings or crowds of people that remind them of their time spent in crowded trenches. According to NBC News, one in every eight soldiers suffers from PTSD.