About 150,000 people suffer a stroke every year. A third make a good recovery within a month. Most however, have long term difficulties with speech or movement and the worst affected die.
Anyone of any age can be affected, though strokes are most common in the elderly. Around 20,000 a year happen in people under 65 and strokes can also affect children and babies.
There are two main types - a bleed in the brain called a haemorrhagic stroke, or a blood clot that blocks a blood vessel in the brain called an ischaemic stroke.
In both cases the effect is to deprive part of the brain of its blood supply, carrying oxygen and nutrients, which causes the affected brain cells to die.
When stroke strikes, fast action is essential to give the patient the maximum chance of surviving unimpaired. But in an elderly person such as Mrs Thatcher, already suffering from Alzheimer's disease, intervention following a stroke might be deemed cruel.
The chances of restoring her to her pre-stroke state would have been extremely slim. It can be kinder to let nature take its course.