A jolt or blow to the head can cause a concussion, a type of traumatic brain injury that about 2.5 million teens, or 15 percent of U.S. high school students, say they sustained during a recent 12-month period while participating in sports or physical activity, according to a report from the Centers for Disease Control and Prevention. About 1 million of those teen athletes reported two or more concussions in that year. Most at risk were those who played on at least one sports team, and the risk rose as the number of teams a student played on increased. About 30 percent of athletes who played on three or more teams had at least one concussion in the preceding year. Although a concussion is usually not life-threatening, it can have a serious effect on thinking, attention, learning and memory, and these effects can worsen and be long-lasting with more than one concussion. Researchers have found that returning to the playing field before full recovery increases the risk for another concussion, and a second concussion can be more severe. Symptoms of a concussion include headaches, dizziness, confusion, memory gaps, trouble concentrating, blurred vision, vomiting and being knocked out. Doctors recommend that anyone with symptoms of a concussion be taken off the field or the court right away and seen by a doctor. Physical activity should be nil until symptoms are gone, followed by a gradual return to play.