Professor Stephen Hawking suffers from amyotrophic lateral sclerosis (ALS), the most common form of motor neurone disease.
What causes it?
ALS, which accounts for more than 90% of all cases of motor neurone disease, causes nerve cells in the brain and spinal cord called motor neurones to degenerate and eventually die.
These cells play an essential role in passing messages to the muscles.
Without them the brain cannot control movement properly, and the muscles cease to work well.
Around one in 10 cases of ALS are thought to be due to genetics, but the trigger for the other 90% of cases remains a mystery.
It is estimated that 3,500 people in the UK have ALS.
What are the symptoms?
As more and more motor neurones are lost, the muscles - particularly in the limbs - begin to waste.
Early symptoms include tripping up when walking, or dropping things.
Twitching and "cramping" of the muscles is also common, especially in the hands and feet.
In the more advanced stages, people often have difficulty speaking, swallowing or breathing and experience paralysis.
Death is usually caused by a failure of the respiratory muscles.
What is the prognosis?
Poor. The average life expectancy for somebody with ALS is just two to five years from the time symptoms first appear.
Half of patients die within 14 months of their diagnosis.
Mel Barry, of the Motor Neurone Disease Association, said: "ALS is quite a complicated and mysterious disease, and often by the time a diagnosis is made people have had symptoms for up to a year."
However, not all people with ALS have the same symptoms, and the rate of progression of the disease can vary greatly
Professor Hawking first developed the disease when he was just 21 years old, and has lived with it for more than 40 years.
The fact that he has lived for long with the condition has been described as remarkable.
It is estimated that only about 5% of people with ALS survive for more than 10 years.
Ms Barry said: "The fact that Professor Hawking has survived for so long is very, very unusual."
Is there any treatment?
Only one drug - Rilutek - is licensed as a treatment for ALS.
The drug works by blocking release of a key chemical called glutamate by the central nervous system, but its effect is limited, extending survival by three to six months.
Trials are currently taking place using another drug, lithium, which has produced promising results in mice, but work is still at an early stage.
Most other treatment is simply palliative, attempting to minimise the effect of the disease.
This can include ventilation systems to help with breathing, feeding tubes if swallowing is a problem and muscle relaxants for muscle cramping.
How was Professor Hawking diagnosed?
On his website, Professor Hawking says that he tries to live as normal a life as possible, and not to think about his condition.
He said his diagnosis came as a great shock.
He had been physically uncoordinated as a child, but had taken up rowing when he went to Oxford.
"In my third year at Oxford, however, I noticed that I seemed to be getting more clumsy, and I fell over once or twice for no apparent reason.
"But it was not until I was at Cambridge, in the following year, that my father noticed and took me to the family doctor.
"He referred me to a specialist and shortly after my 21st birthday I went into hospital for tests."
What course has his condition taken?
Professor Hawking was able to feed himself and get in and out of bed until 1974.
Until that point he and his wife were able to manage without outside help, but then had to rely on live-in help from one of his research students.
In 1980, he changed to a system of community and private nurses, who came in for an hour or two in the morning and evening.
This lasted until he caught pneumonia in 1985, and had to have a tracheotomy operation.
After this, he needed 24-hour nursing care.
Before the operation, his speech had been getting more slurred, so that only a few people who knew him well could understand him.
However, he could communicate. He wrote scientific papers by dictating to a secretary, and gave seminars through an interpreter.
The tracheotomy operation removed his ability to speak altogether, and he had to rely on a small portable computer and a speech synthesizer fitted to his wheel chair.