It’s not unusual for people who have experienced traumatic events to have flashbacks, nightmares, or intrusive memories when something terrible happens -- like the explosions at the Boston Marathon.
Be tolerant of your nervous system: It’s having a normal reaction. Try not to get hooked to news reports, which may seem particularly compelling. Spend time with loved ones in favorite activities or outside in nature, and avoid alcohol.
Learn more below, including how to help children.
Posttraumatic stress disorder, or PTSD, is a serious potentially debilitating condition that can occur in people who have experienced or witnessed a natural disaster, serious accident, terrorist incident, sudden death of a loved one, war, violent personal assault such as rape, or other life-threatening events.
Most people who experience such events recover from them, but people with PTSD continue to be severely depressed and anxious for months or even years following the event. Learn about PTSD symptoms .
Women are twice as likely to develop posttraumatic stress disorder as men, and children can also develop it. PTSD often occurs with depression, substance abuse, or other anxiety disorders.
Trauma survivors who have PTSD may have trouble with their close family relationships or friendships. Their symptoms can cause problems with trust, closeness, communication, and problem solving, which may affect the way the survivor acts with others. In turn, the way a loved one responds to him or her affects the trauma survivor. A circular pattern may develop that could harm relationships. Read more from the National Center for PTSD. 
PTSD brochure