Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after exposure to one or more terrifying events that threatened or caused grave physical harm.
It is a severe and ongoing emotional reaction to an extreme psychological trauma. This stressor may involve someone's actual death, a threat to the patient's or someone else's life, serious physical injury, or threat to physical or psychological integrity, overwhelming psychological defenses.
In some cases it can also be from profound psychological and emotional trauma, apart from any actual physical harm. Often, however, the two are combined.
PTSD is a condition distinct from traumatic stress, which has less intensity and duration, and combat stress reaction, which is transitory. PTSD has also been recognized in the past as railway spine, shell shock, battle fatigue, traumatic war neurosis, or post-traumatic stress syndrome (PTSS).
Diagnostic symptoms include reexperience such as flashbacks and nightmares, avoidance of stimuli associated with the trauma, increased arousal such as difficulty falling or staying asleep, anger and hypervigilance. Per definition, the symptoms last more than 6 months and cause significant impairment in social, occupational, or other important areas of functioning (e.g. problems with work and relationships.)
In the series
In the Resident Evil 5 Viral Campaign, Chris Redfield appears to be suffering from a form of PTSD after Kijuju. He appears to have a flashback each time a bright light flashes him. In Resident Evil 6, Chris seems to be suffering from a form of PTSD after the death of several men under his command during a mission in Edonia. Piers Nivans finds him in a bar, heavily intoxicated and lacking any memory of his men, the BSAA or even bio-terrorism in general.