A concussion is an invisible brain injury caused by a blow to the head that cannot be detected by scans. Symptoms include memory loss, headaches, dizziness, and nausea. Concussions can negatively affect players long after they retire from the sport and frequently lead to long-term health problems. While precautions like improved equipment, rules against blindside hits, and education are in place, concussions remain common due to the physical nature of contact sports and the difficulty of controlling every action that occurs during games.