“He has the most common form called amyotrophic lateral sclerosis and there is no cure.
Symptoms start in the hands and feet and muscles often become stiff as well as weak at first. The electrical signals from the brain and spinal cord to the muscles become damaged and eventually stop, although the reason why is unclear.
One or two people in every 100,000 people in the UK have this form of motor neurone disease, around 5,000 people in total.
It is rare to develop the condition under the age of 40, as Prof Hawking did. It usually begins between the ages of 50 and 80.
Progression varies and about seven in ten people die within five years of diagnosis, with only one in ten living for more than ten years.
Intellect, the senses and emotions are not affected, leading many to surmise that the condition is like being trapped, however Prof Hawking says this opinion is contemptible.
The cause of death is usually respiratory failure combined with a chest infection as the muscles needed for breathing become weakened.
On his website Prof Hawking describes coxing and rowing during his time at Oxford University but gradually becoming more clumsy.
He was admitted to hospital shortly after his 21st birthday after he had moved his studies to Cambridge, and although he was not told then what the condition was he knew it was incurable.
Even though his condition to worsen slowly he got married to Jane Wilde and had three children.
Up until 1974 Prof Hawking was able to feed himself and get in and out of bed alone but was already using an electric wheelchair that was to become a hallmark of his.
In 1985 he caught pneumonia and had to have a tracheotomy, which is a hole made in his windpipe to breathe through, and after that required 24 hour nursing care which was paid for by grants from foundations.
Before the tracheotomy Prof Hawking’s speech was becoming slurred but afterwards he was unable to speak at all and the only way he could communicate was by raising his eyebrows to indicate letters as they were held up on cards.
Then a computer expert in America sent him the software for the Equaliser programme which allowed him to choose words from a menu, map out what he wanted to say and then send it to a speech synthesizer.
The speech synthesizer has been developed and now allows Prof Hawking to communicate more rapidly, however he jokes, that it gives him an American accent.”