Concussions are traumatic brain injuries that affect brain function. Effects from these injuries are often temporary but can include headaches, problems with concentration, memory, balance and coordination. Concussions are usually caused by a blow to the head or violent shaking of the head and upper body. Some concussions cause loss of consciousness, but most do not. And it is possible to have a concussion and not realize it. Concussions are common in contact sports, such as football. However, most people gain a full recovery after a concussion.