“PTSD is a medical diagnosis, established in 1980, defining symptoms that last at least a month after experiencing a major trauma. These symptoms include remembering or reliving the trauma when you do not choose to; feeling numb and withdrawn; and, having forms of anxiety that interfere with daily life.”
The mechanisms by which severe trauma can induce PTSD are still unclear.
“Increasing levels of stress produce an inverted U-shaped effect on memory retention. Humans experiencing an acute trauma may develop posttraumatic stress disorder (PTSD), but the key contributions of trauma to PTSD formation are still unknown.”
Further, some individuals go on to develop PTSD whilst others (exposed to the same stressing situations) do not :
“Stress induces a shift from hippocampus-based “cognitive” toward dorsal striatum-based “habitual” learning and memory. This shift is thought to have important implications for stress-related psychopathologies, including post-traumatic stress disorder (PTSD). However, there is large individual variability in the stress-induced bias toward habit memory, and the factors underlying this variability are completely unknown.”