An unconscious person is completely unaware of what is going on and is unable to make purposeful movements. Fainting is a brief form of unconsciousness; a coma is a deep, prolonged state of unconsciousness.

Causes of unconsciousness include stroke, epilepsy, heat stroke, diabetic coma, insulin shock, head injury, suffocation, alcohol or drug overdose, shock, bleeding, irregular heartbeat, and heart attack.

Fainting is a short-term loss of consciousness, usually lasting only a few seconds. It is most often caused by a momentary drop in blood flow to the brain. When you fall or lie down, blood flow is improved and you regain consciousness. Fainting is a mild form of shock and is usually not serious. If it happens often, there may be a more serious problem. Dizziness and fainting can also be brought on by sudden emotional stress or injury. See Dizziness and Vertigo.