A person who is unconscious is not aware of what is going on around him or her. He or she may not be able to make purposeful movements.
A person may become unconscious from an injury or a health condition.
Fainting or a seizure disorder (epilepsy) may cause unconsciousness that is usually brief.
Heart problems, such as stroke, heart attack, or changes in heart rate or rhythm (arrhythmia), can block blood and oxygen to the brain and cause unconsciousness.
Lack of adequate oxygen, such as when there is too much carbon monoxide in the air a person breathes, can cause a gradual unconsciousness.
Head injuries can "knock out" a person, making him or her unconscious.
Any event that leads to being in a coma, which is a deep, prolonged state of unconsciousness. Diabetic coma, caused by very high or very low blood sugar, is one type of coma.
Heavy alcohol or drug use or problems caused by quitting their use (withdrawal) can cause the body to go into a state of shock that may cause unconsciousness. Heatstroke, an injury, or a traumatic event can also cause shock and unconsciousness.