Post Traumatic Stress Disorder
This is somewhat like a damaged "flight-or-fight" response. It's when people are no longer in harmful or scary situations, but their "fight or flight" kicks in as if they are.
PTSD is caused when someone goes through or witnesses a traumatic situation. It's not genetic, it can happen to anyone of any age.
This affects your amygdala, hippocampus, and prefrontal cortex. These are the main areas of the brain that deal with stress response, and can be changed or damaged for life if affected by PTSD.
PTSD usually kicks in when someone is reminded in any way of the trauma that caused their PTSD. It could be remembering it, hearing something that sounded like the trauma, seeing someone that was at the trauma, or even just being stressed or irritated for another reason.
PTSD can be treated with certain types of therapy and medications.
PTSD can be anywhere from hardly noticeable, to completely changing a person. PTSD can include things like depression, and cause a person to not act anything like they used to at first.