A stroke is a condition caused by the inability of blood to reach any part of the brain, either in whole or in part. It is usually caused by an infarction in one of the major arteries that supply blood to the brain, either a blood clot or a loose piece of plaque.

Stroke has been recognized as a medical phenomenon since the time of Hippocrates, the father of medicine in 400 BC. He called it "apoplexia", meaning "struck down by violence". The modern term has been in use for centuries, with a doctor in 1820 writing "The term Apoplexia was employed by the Greeks, and is still used, to denote a disease in which the patient falls to the ground, often suddenly, and lies without sense or voluntary motion. Persons, instantaneously thus affected, as if struck by lightning."

A stroke is an immediate life threatening condition and is invariably fatal if untreated. Patients who survive a stroke may nevertheless be stricken by localized brain damage, which can affect the ability to walk or speak, although cognitive abilities may remain intact. However, a minor stroke may have no lasting symptoms.

The usual treatment for a stroke is blood thinners to break up any clots that may be occluding blood flow to the brain. Although in the past any obvious symptoms of a stroke inevitably led to brain damage, modern treatment applied in a timely fashion can usually prevent this from happening.

High blood pressure is a contributing factor in the risk of stroke.

Early symptoms of a stroke include confusion, aphasia, and light headedness. Persons exhibiting these symptoms should be receive immediate medical treatment.

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