A stroke occurs when the flow of blood to part of the brain is disrupted. When this is due to a blockage of one of the arteries that supply the brain with blood, it is called an ischemic stroke. Arteries can become blocked when the inner lining of the blood vessel is damaged, allowing a blood clot to form around the diseased area. As other cells clump around the blood clot in an effort to patch up the damage, the blood vessel becomes progressively narrower until little or no blood can get through.

Less often, a stroke occurs when an artery bursts and blood leaks into nearby brain tissues. This is called a hemorrhagic stroke. If the bleed occurs beneath the arachnoids membrane (one of three layers of tissue protecting the brain) it is called a sub-arachnoids hemorrhage.

Starved of blood or swamped in it, nerve cells quickly start to die. Dying cells release chemicals that destroy neighboring cells in a cascade effect, leading to varying amounts of brain damage.