Life expectancy generally ranges from two to five years after symptoms set in

Stephen Hawking may be no Sir Isaac Newton, but he is extraordinary. In particular, he should have been dead long long ago:

British scientist Stephen Hawking has decoded some of the most puzzling mysteries of the universe, but he has left one mystery unsolved: How has he managed to survive so long with such a crippling disease?

The physicist and cosmologist was diagnosed with Lou Gehrig's disease -- also called amyotrophic lateral sclerosis, or ALS -- when he was 21. Most people die within a few years of the diagnosis. Today, Hawking turns 70.

"I don't know of anyone who's survived this long," said Ammar Al-Chalabi, director of the Motor Neurone Disease Care and Research Centre at King's College London. He does not treat Hawking but called his longevity "extraordinary."

Hawking achieved all that despite being nearly entirely paralyzed and in a wheelchair since 1970. He now communicates only by twitching his right cheek. Since catching pneumonia in 1985, he has needed around-the-clock care and relies on a computer and voice synthesizer to speak.

A tiny infrared sensor sits on his glasses, hooked up to a computer. The sensor detects his cheek pulses, which select words displayed on a computer screen. The chosen words are then spoken by the voice synthesizer. It can take up to 10 minutes to formulate a sentence.

"The only trouble is (the voice synthesizer) gives me an American accent," the Briton wrote on his website.