Brain death occurs when parts of the brain stem that control breathing, circulation and blood pressure, irreversibly stop working. This prevents oxygen from being delivered to the vital organs, without which they cannot survive.

Sometimes a body’s vital functions can slow down so much that they are mistaken for being brain dead. In the West African Vodun religion a zombie is believed to be a dead person taken from the grave and held as a slave by a bokor (wizard). One theory is that the bokor uses a poison from the deadly puffer fish, which can paralyse and produce coma, so the 'dead' person only appears to be brought back as a zombie.

Today, doctors test for electrical activity in the brain stem. Usually, if there is none, brain death has occurred. However, even when there has been no activity for many hours, as long as the heart is pumping, the person can remain alive.