It is common to experience distressing memories, difficulty sleeping, and restlessness following a tragedy; however, for most people these reactions tend to improve with time. If improvement does not occur or if the reactions worsen, it may be an indicator of Post-Traumatic Stress Disorder (PTSD). PTSD is characterized by four symptom clusters: Persistent mood disturbances, hypervigilance, re-experiencing, and avoidance. Not everyone who experiences a traumatic event will develop PTSD. About 60% of women and 50% of men experience at least one traumatic event in their lifetime, and most will never develop PTSD. PTSD affects approximately 1 in 15 children, 1 in 9 adult women, and 1 in 18 adult men in the United States.