A concussion is an injury to the brain that causes changes in brain function. It usually is caused by a blow to the head, but can also be due to severe whiplash. Concussions are relatively common, with at least four million new concussions diagnosed each year in the United States. Most concussions are mild, and people usually recover fully. However, a percentage of people (between 20%-30%) do not properly recover from their concussion and are left with lingering symptoms for an extended period of time. Up to 88% of concussion cases might go unrecognized according to the research.