Post-Traumatic Stress Disorder (PTSD) occurs in some individuals who have been through a harrowing experience in which they either felt in imminent danger of getting seriously injured or killed, or they witnessed another person or persons get seriously injured or killed.
Symptoms include: recurrent, distressing nightmares of the incident; flashback, in which something triggers them to re-experience the incident, while awake, to where they momentarily believe they are actually re-living it; irritability; feeling unsafe; and others.
What PTSD is not, is weakness. Anyone, given the right circumstances, may be prone to developing PTSD. It originates, in part, in that part of our brain that alerts us to danger and prepares us to either run or fight our way to safety. Military members, police and other First Responders are at particular risk because of the nature of their occupations.
With proper treatment, such as Eye Movement Desensitization and Reprocessing (EMDR) and other exposure techniques, many PTSD sufferers see a reduction and even elimination of symptoms.