If they don't get the oxygen and glucose they require, your brain cells start to die in minutes. That's exactly what happens when someone has a stroke. Most strokes are ischemic strokes, usually caused by atherosclerosis. Fatty deposits form on the walls of the arteries, which stiffen and become narrower. The narrowed arteries can easily become blocked completely by blood clots, cutting off the brain's vital blood supply. Stroke is the number two leading cause of death worldwide, second only to heart disease.