Extreme reactions to a highly stressful event constitute posttraumatic stress disorder (PTSD). Examples of a stressful event would be being raped, robbed, or assaulted; escaping from a flash flood; or being in military combat. Stress reactions last for months or years and often include physiological symptoms such as difficulty in sleeping or concentrating. Individuals with PTSD may re-experience the event through nightmares or images that remind them of the event. Another aspect of PTSD is attempting to avoid feeling or thinking about the trauma or event.