Atherosclerosis is a degenerative condition in which arteries build up deposits called plaques (atheromas) which consist of lipids (mainly cholesterol), connective tissue and smooth muscle cells originating from the arterial wall. Plaques develop quietly over a period of years and are unnoticeable until there is an interruption in the normal flow of blood. Plaques may partially or totally block the blood's flow through an artery. Two things that can happen where plaques occur are: bleeding (hemorrhage) into the plaque; and formation of a blood clot (thrombus) on the plaque's surface. Atherosclerosis affects large and medium-sized arteries. The type of artery and where the plaque develops varies with each person. Atherosclerosis research has witnessed startling progress in recent years, partially due to new drugs as well as to new breakthroughs in molecular medicine.