Image caption Prof Hawking communicates using a clever computer programme that was specially designed for his needs

Few would contest that Prof Stephen Hawking is an incredible man.

The former Lucasian professor of mathematics and theoretical physics at the University of Cambridge is widely regarded as one of the most brilliant theoretical physicists since Einstein.

He has 12 honorary degrees, a CBE and in 2009 he was awarded the Presidential Medal of Freedom, the highest civilian award in the US.

The human race is so puny compared to the universe that being disabled is not of much cosmic significance. Prof Hawking

But aside from his academic achievements, the professor is also something of a medical marvel.

Now aged 70, he has long defied and baffled medical experts who predicted he had just months to live in 1963 when he was diagnosed with Motor Neurone Disease (MND).

Only 5% of people with the form of MND that he has - a condition called amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease - survive for more than a decade after diagnosis.

Most die within a few years of diagnosis.

The fact that Prof Hawking has lived for nearly half a century with a condition that progressively attacks the nerves serving the muscles of the body has been described as remarkable.

Incurable

The man himself says: "I have been lucky, that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope.

"I am quite often asked: 'How do you feel about having ALS?'

Motor Neurone Disease Five percent of cases are hereditary - in the remaining 95%, the cause is unknown

Around 5,000 people in the UK have MND and it kills five people every day

With no cure half die within 14 months of diagnosis

Just one licensed treatment for MND called Riluzole, which can extend survival for up to six months

"The answer is, not a lot. I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many."

Doctors are still in the dark about what causes motor neurone disease.

About 5% of cases are hereditary, meaning they are passed down from one generation to the next.

But the rest of the time, it is a mystery why someone who has previously been fit and well develops this debilitating disease.

Prof Hawking started having symptoms shortly before his 21st birthday. At first they were mild - a bit of clumsiness and few unexplained stumbles and falls.

But, predictably, by the very nature of the disease, his incurable condition worsened.

The diagnosis came as a great shock, but also helped shape his future.

"Although there was a cloud hanging over my future, I found, to my surprise, that I was enjoying life in the present more than before.

"I began to make progress with my research, and I got engaged to a girl called Jane Wilde, whom I had met just about the time my condition was diagnosed.

"That engagement changed my life. It gave me something to live for."

Up until 1974, Prof Hawking and his wife Jane, who by that time had three children together, managed his ALS largely on their own.

Key points in life and work Born in Oxford on 8 Jan 1942, brought up in St Albans. Father a research biologist, mother a radical free-thinker

Labelled a swot at public school, he liked horse riding, rowing, classical music and debating

A Brief History of Time - the 1988 layman's guide to cosmology - has sold 10 million copies worldwide

Discovered "Hawking radiation", where black holes leak energy and fade to nothing, and "theory of everything" suggesting universe evolves according to well-defined laws

Popular ambassador for science, he lent his synthesised voice to various recordings and appeared in BBC comedy Red Dwarf, The Simpsons and Star Trek

In 2001, he claimed mankind could be wiped out by a genetically engineered "doomsday" virus

In an interview with the New Scientist ahead of his 70th birthday, he said he spent most of the day thinking about women, who he says are "a complete mystery"

He was still able to feed himself and get in and out of bed, although walking any great distance was not possible.

But as things got more difficult and his muscles began to fail him, he and his wife decided to take in one of Prof Hawking's research students to live with them.

In exchange for help around the house, they were given free accommodation and personal tuition by the professor.

Over the next few years it became clear that the family would need professional nursing assistance and Professor Hawking would be spending most of his waking life in a wheelchair.

Then, in 1985, Professor Hawking developed a serious complication. He caught pneumonia.

His lungs, already weakened by his ALS, struggled to cope with the respiratory infection.

Often, this kills patients with MND, but Prof Hawking fought back.

To help with his breathing difficulties he needed an operation called a tracheostomy, where a tube is placed into the windpipe through the neck, effectively bypassing the mouth and nose.

The treatment was a success, but it irreversibly removed his voice and meant he would need around the clock care from a dedicated team for the rest of his life.

The voice

For a time, the only way Prof Hawking could communicate was to spell out words letter by letter, by raising his eyebrows when someone pointed to the right letter on a spelling card.

Image caption Prof Hawking is a highly recognisable figure in popular culture as well as the scientific world.

But then a computer expert in California, called Walt Woltosz, heard of Prof Hawking's plight and sent him a computer program he had written, called Equalizer.

This allowed Professor Hawking to select words from a series of menus on the screen, controlled by a switch in his hand.

This was coupled to the speech synthesiser that has become Professor Hawking's trademark voice.

Prof Hawking says: "One's voice is very important. If you have a slurred voice, people are likely to treat you as mentally deficient: 'Does he take sugar?' This synthesiser is by far the best I have heard, because it varies the intonation, and doesn't speak like a Dalek.

"The only trouble is that it gives me an American accent."

More recently, patients with MND are beginning to be offered the option of voice banking - stockpiling recordings of themselves before their speech declines, so that when they need to use a computerised system it will be their own voice rather than a computerised version that is used.

This is not an option for Prof Hawking though.

Facing the future

His life, like anyone's, has consisted of highs and lows. He and Jane divorced in 1990 after 26 years of marriage.

Five years later, he married again - this time to one of his nurses.

Image caption Prof Hawking experienced what space travel feels like during a trip in a zero gravity simulator

But this 11-year marriage also ended in divorce, amid allegations that Prof Hawking had been the victim of assault during the relationship - something the professor has denied.

Police investigating the case said they could find no evidence to back the claims.

A consistent thing in his life has been his work.

And at the age of 70, Professor Hawking shows no sign of slowing down on that front.

Although he has now stepped down from the Lucasian chair after a historic 30 years, he continues working at the University of Cambridge and recently published a new book - The Grand Design.

Tributes from those who know him He has managed not just to take ordinary people out to the frontiers of science, but he takes them there as a huge adventure. It's fun for him and he insists it be fun for everybody else. This is a huge contribution. Kitty Ferguson, biographer

It's an inspiration that he's been able to control his world and make genuine scientific contributions under very difficult situations. Frank Wilczek, Massachusetts Institute of Technology

You can always tell when he's cracking a joke because his face will light up. He likes to be provocative, what he will say will provoke people to respond. Gary Gibbons, professor of theoretical physics, Cambridge University

The grandfather-of-three continues to seek out new challenges.

He recently experienced first-hand what space travel feels like by taking a zero-gravity flight in a specially modified plane.

With regards to his physical health, he remains optimistic.

"The human race is so puny compared to the universe that being disabled is not of much cosmic significance.

"I'm sure my disability has a bearing on why I'm well known. People are fascinated by the contrast between my very limited physical powers, and the vast nature of the universe I deal with.

"I'm the archetype of a disabled genius, or should I say a physically challenged genius, to be politically correct. At least I'm obviously physically challenged. Whether I'm a genius is more open to doubt."