Posttraumatic stress disorder, or PTSD, is a serious potentially debilitating condition that can occur in people who have experienced or witnessed a natural disaster, serious accident, terrorist incident, sudden death of a loved one, war, violent personal assault such as rape, or other life-threatening events. Research has recently shown that PTSD among military personnel may be a physical brain injury, specifically of damaged tissue, caused by blasts during combat. Most people who experience such events recover from them, but people with PTSD continue to be severely depressed and anxious for months or even years following the event. The disorder is characterized by three main types of symptoms: Re-experiencing the trauma through intrusive distressing recollections of the event, flashbacks, and nightmares. Emotional numbness and avoidance of places, people, and activities that are reminders of the trauma. Increased arousal such as difficulty sleeping and concentrating, feeling jumpy, and being easily irritated and angered.