What originally was described as “shell shock” by the military, PTSD is the constellation of symptoms due to trauma (war experience, physical violence, or other events), creating a psychological and perceptual alteration. PTSD is characterized by depression, anxiety, flashbacks and recurrent nightmares, with active avoidance of reminders of the event. Because of PTSD’s generalizing character, psychotherapy and anti-anxiety medication can be extremely helpful in treating the effects. Very recent research suggests that some of the symptoms (e.g., jumpiness aspect of anxiety) may stem from an “... alteration of genes, induced by a traumatic event, changes a person's stress response and leads to the disorder," said researcher Sandro Galea, MD.
http://www.merriam-webster.com/medical/post-traumaticstressdisorder