From the National Center for PTSD, US Department of Veterans Affairs. http://www.ptsd.va.gov/public/pages/what-is-ptsd.asp
Post Traumatic Stress Disorder (PTSD) can occur after you have been through a traumatic event. A traumatic event is something terrible and scary that you see, hear about, or that happens to you, like: combat exposure, child sexual or physical abuse, terrorist attack, sexual or physical assault, serious accidents like a car wreck or natural disasters like a fire, tornado, hurricane, flood or earthquake.
During a traumatic event, you think that your life or others’ lives are in danger. You may feel afraid or feel that you have no control over what is happening around you. Most people have some stress-related reactions after a traumatic event, but not everyone gets PTSD. Fear, sadness, guilt, anger and sleep problems are common. You may have bad memories of the event. If your reactions don’t go away over time and they disrupt your life, you may have PTSD.
From the National Center for PTSD (US Veterans Affairs) booklet Understanding PTSD