Concussion. A concussion is. What is a concussion? There are many. What causes a concussion? Every time you. How Dangerous Are Concussion. Over time, repeated concussions can change chemicals and cells in the brain. You don't have to pass out (lose consciousness) to have a concussion. What are the symptoms. Symptoms of a concussion range from mild to severe and can last for hours. How long takes a concussion. Your doctor will. How Concussion Is Treated.