Many of us will experience some kind of trauma during our lifetime. Sometimes, we escape with no long-term effects. But for millions of people, those experiences linger, causing symptoms like flashbacks, nightmares, and negative thoughts that interfere with everyday life. Joelle Rabow Maletis details the science behind post-traumatic stress disorder, or PTSD. Lesson by Joelle Rabow Maletis, directed by Tomás Pichardo-Espaillat.