The most common stroke is the result of an embolus. As the body ages, atherosclerosis begins to attacks the vessels, meaning that plaque builds up with them, to which the blood cells adhere and clump together, forming a clot. This clot then breaks off from the wall of the vessel and travels to the brain, where the vascular system is too small to accommodate it. The clot blocks off circulation to that part of the brain, and the brain then suffers ischemic stroke, resulting in loss of function and paralysis. 

Risk factors for an ischemic stroke often include smoking, high blood pressure, excessive intake of alcohol, and a history of atherosclerotic vessels in which blood clots are allowed to form. Diets high in cholesterol have been found to enhance the process of artery clogging, including the carotid artery, one of the main suppliers to the brain. Atrial fibrillation is also considered to be a risk factor as the blood that continues to stagnate in the atria form clots that will enter the systemic circulation via venous thrombosis, which forms in the vein but finds its way into the arterial circulation. 

A hemorrhagic stroke is usually the result of an abnormality in anatomy structure, specifically an arteriovenous malformation, which is very prone to rupture and bleeding. It is usually a silent defect, many never knowing that it is there until the structure begins to leak. Because the brain tissue is highly sensitive, the excess swelling causes the stroke, from which many do not recover. 

Moyamoya disease is also a risk factor. This is a disease that constricts the arteries in the brain, inhibiting blood flow and resulting in both transient ischemic attacks, often called warning strokes, and the more frequent ischemic strokes. Treatment for this type of disease is usually surgical, but left untreated the condition proves fatal. 

An MRI is a frequently used tool to diagnose that a stroke has occurred, and the effects of the disease are reduced if the sufferer is seen early enough to dissolve the clots, either by anti-platelet drugs of thrombolytic agents such as Heparin. Should the MRI reveal hemorrhage, a decompression surgery can be performed to stop the bleeding inside the brain and, hopefully, limit the disabilities, such as paralysis and speech difficulties. 

Call early, call often is the general practice for anyone believed to be having a stroke. It is not so much that the brain cells are lost forever, but that they take so much longer than the rest of the system to recuperate, and recognizing the signs of a stroke early often carries a better prognosis for life with only limited disability.