A gradual narrowing of the coronary arteries is preferable as this gives time to alternative arteries ( called collateral arteries) to become functional, and supply blood to the deprived parts.
Sometimes, a small plaque may rupture inside the lumen of the coronary artery due to high BP, stress, the effect of nicotine in tobacco, heavy exercise or extreme excitement. The normal inner lining of the blood vessel is lost which encourages blood clot formation and the artery may be completely blocked suddenly. Such events lead to heart attacks, many of which are severe.
Although, Atherosclerosis cannot be prevented its progress can be slowed by maintaining the blood cholesterol levels strictly within normal limits. This can be done by diet control, exercise and medicines.