An aneurysm is a bulge in a blood vessel caused by a weakness in the blood vessel wall, usually where it branches off.
As blood passes through the weakened area, the pressure causes a small area to bulge outwards like a balloon.
Aneurysms can develop anywhere in the body, but the two most common places are in the abdominal aorta - the artery that transports blood away from the heart - and the brain.
Most brain aneurysms will only cause noticeable symptoms if they burst - this causes a haemorrhage, where bleeding can lead to extensive brain damage.
Other symptoms include a sudden, agonising headache, similar to a sudden hit on the head. Stiffness in the neck, sickness and pain are also signs.
Smoking, a poor diet and lack of exercise can increase the risk of an aneurysm.
About three in five people who suffer a brain haemorrhage will die within two weeks.
Anybody who suspects that someone has developed a ruptured aneurysm should call 999 immediately.