Concussion, the most common among traumatic brain injuries, which occurs 1.7 million times a year in the U.S., represents a major public-health problem. It occurs when there is a sudden acceleration or deceleration of the head, a process depicted here in this animation.

A blow can produce a brief loss of consciousness, headaches and impaired cognition, among other symptoms. Symptoms can last for days or sometimes longer. And a person who experience one risks another and may find recovery takes longer.

Scientists continue to learn more about the nefarious consequences of repeated concussions. In the February issue of Scientific American, writer Jeffrey Bartholet details in “The Collision Syndrome evidence for yet another neurodegenerative disorder that can result from concussions.

Interactive by Ryan Reid