Atherosclerosis begins in childhood with the appearance of aortic fatty streaks. Aortic fatty streaks of some degree are present in practically all individuals from every human population that has been studied. The average amount of aortic intimai surface involved with fatty streaks does not differ much among human populations. Coronary fatty streaks begin to form in adolescence. It is unusual for children or teenagers to have a heart attack or stroke because of atherosclerosis. This is because vessel narrowing, caused by atherosclerosis, takes many years to develop. However, the process of atherosclerosis begins in childhood. For most children, atherosclerosis is mild and progresses slowly. In some children, atherosclerosis worsens rapidly, increasing the risk of heart disease, and less commonly, stroke in early adult life. This conversion takes place at earlier ages in populations with high morbidity and mortality from coronary heart disease.