Posttraumatic stress disorder (PTSD) is a severe, sometimes disabling, anxiety disorder that can develop after a potentially traumatic event involving actual or threatened death, serious injury, or sexual violation. The diagnosis of PTSD requires symptoms for at least one month from three categories: re-experiencing, avoidance, and increased arousal. In contrast to an acute response to trauma, the stress reactions of persons who develop PTSD do not resolve quickly; symptoms can last for long periods of time and may increase in severity.