is generally considered positive, with total resolution of symptoms in many, but not all, cases. For 50% of people, post-concussion symptoms go away within a few days to several weeks after the original injury occurs. In others, symptoms may remain for three to six months, but evidence indicates that many cases are completely resolved within six months. The majority of symptoms are largely gone in about half of people with concussion one month after the injury, and about two thirds of people with minor head trauma are nearly symptom-free within three months. Persistent, often severe headaches are the longest lingering symptom in most cases and are the most likely symptom to never fully resolve. It is frequently stated in the literature and considered to be common knowledge that 15–30% of people with PCS have not recovered by a year after the injury, but this estimate is imprecise because it is based on studies of people admitted to a hospital, the methodologies of which have been criticized. In approximately 15% of people, symptoms may persist for years or be permanent. If symptoms are not resolved by one year, they are likely to be permanent, though improvements may occur after even two or three years, or may suddenly occur after a long time without much improvement. Older people and those who have previously had another head injury are likely to take longer to recover. The way in which children cope with the injury after it occurs may have more of an impact than factors that existed prior to the injury. Children's mechanisms for dealing with their injuries may have an effect on the duration of symptoms, and parents who do not deal effectively with anxiety about children's post-injury functioning may be less able to help their children recover. If another blow to the head occurs after a concussion but before its symptoms have gone away, there is a slight risk of developing the serious second-impact syndrome (SIS). In SIS, the brain rapidly swells, greatly increasing intracranial pressure. People who have repeated mild head injuries over a prolonged period, such as boxers and Gridiron football players, are at risk for chronic traumatic encephalopathy (or the related variant dementia pugilistica), a severe, chronic disorder involving a decline in mental and physical abilities. Epidemiology It is not known exactly how common PCS is. Estimates of the prevalence at three months post-injury are between 24 and