Six million die from them and five million people suffer long-term effects, including loss of mobility and speech, after having a stroke.
The haemorrhagic stroke suffered by former Israeli president Shimon Peres is one of the most serious forms of stroke and can be fatal.
Peres, 93, is the last of the state of Israel's founding fathers and has held nearly every major office in the country, including prime minister twice and president.
Doctors have described his condition as critical but stable.
Here are some key facts on strokes:
Main types of stroke
There are two main types of stroke -- one caused by blockages of vessels (ischaemic) and the other by bleeding (haemorrhagic). The brain needs a constant supply of blood to work properly and blood clots, or blockages, cause 70 to 80 percent of all types of stroke.
When the blood supply is cut off, it causes the brain to become damaged or die. A bleeding stroke is rarer and more likely to be fatal.
What is a hemorrhagic stroke?
More than 15 million people suffer cerebral haemorrhagic strokes in the world every year, according to the World Heart Foundation.
Six million die from them and five million people suffer long-term effects, including loss of mobility and speech, after having a stroke. There is also a high risk of suffering a repeat stroke.In a cerebral hemorrhage, an artery inside the brain bursts, leaking blood into the brain tissue.
It can be caused by a head injury or by a weakened artery wall, which bulges out to form a blood-filled pouch -- an aneurism -- that then bursts.
Sometimes a blood vessel on the surface of the brain bursts and leaks into the space between the brain and the skull, producing an extremely dangerous condition known as a subarachnoid haemorrhage.
What happens to the brain in a hemorrhagic stroke?
The loss of constant blood supply stops some brain cells working. Accumulated blood flooding into the brain tissue can damage the functioning of the brain.
What are the outcomes?
The crucial factor in recovery is the amount of bleeding. Often, people die from a hemorrhagic stroke because of the pressure on their brains.
But those who pull through tend to recover better than those who suffer a clot-type stroke because the brain can regain some lost functions as the pressure is relieved.
A vessel blocked by a clot, however, kills off part of the brain permanently.