As the name implies, posttraumatic stress disorder occurs only after (post) an extremely stressful event (trauma). The more severe the trauma and the longer the person is exposed to it, the greater the likelihood of developing PTSD.

PTSD is only diagnosed after a) a person has been exposed to an extreme trauma, b) symptoms develop that last at least one month and c) the symptoms create extreme distress and dysfunction. Three or more of the following characteristics are usually present:

• numbing, detachment or absence of emotional response
• reduced awareness of surroundings (being dazed)
• sensation that surroundings are distorted or unreal
• the feeling that you are different, strange or unreal
• an inability to remember parts of the trauma.

In addition to three or more of these five characteristics, the traumatic event is relived repeatedly. This can take the form of recurrent images, thoughts, dreams or "flashbacks" of the event. Even reminders of the event can cause extreme distress, so many people go out of their way to avoid places or events that resemble the traumatic event in some ways. Many experience increased anxiety, restlessness, sleeplessness, irritability, poor concentration, hypervigilance or an exaggerated startle response. Some are even plagued by feelings of "survivor's guilt," because they survived when others did not or because of certain things they may have had to do to survive. This complete set of symptoms is obviously very disruptive and stressful to the victim as well as their family and loved ones. It can even impair job performance and social functioning.