A concussion is the most common brain injury in children according to a wikipedia page dedicated to head injuries.  Most of these concussions are caused from a blow to the head during a fall, car accident or sporting event.  If left untreated or if the injured person goes back to a risky activity too soon, it could be fatal.

Second Impact Syndrome happens when a person receives a concussion and then receives another concussion before the first one has time to heal.  A concussion is just a swelling of the brain.  This swelling puts pressure on sensitive areas as it presses against the skull.  A person who receives a second impact to the head, before it has had time to heal could be susceptible to a TBI (traumatic brain injury) or even death.  Young people are especially at risk since their brains are more sensitive to the swelling.

A teenage boy died to to SIS during a football game.  The initial blow to the head happened 3 weeks earlier.  The boy complained of headaches during his time off but was still cleared to play.  The full story can be read at http://www.nytimes.com/2008/10/17/sports/17preps.html?hp.

It is important to protect your head at all times.  Buckle up, where a helmet and other safety gear when possible.  If you do receive a blow to the head, make sure you see a physician.  Let them determine when it is ok for you to participate in physical activities again.