According to the National Institute of Mental Health, post-traumatic stress disorder (PTSD) develops after a terrifying event that involves physical harm or threat of physical harm. This can be from direct exposure, witnessing a traumatic event or from someone close to you experiencing a traumatic event. PTSD is classified by mental health professionals as an anxiety disorder. Those at risk include veterans, victims of crime, kidnap victims and survivors of car accidents, bombings or natural disasters. PTSD affects 7.7 million American adults.