For a 67 year old man with ALS who has already defied medical science, this is not good news at all. Remember what happened to Christopher Reeve. When you are in a condition like this, even otherwise normal ailments may become life-threatening.
I have been recently reading a lot about Hawking in Leonard Susskind's splendid book "The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics". The rather grandiose title of the book obscures a perfectly entertaining and informative romp through the world of black holes; this is about as close as possible to black hole thermodynamics, string theory and quantum mechanics that we laymen can get without being drowned in a whirlpool of math. Susskind who is a professor at Stanford tells the story of the paradox of information falling into a black hole and supposedly disappearing with lots of verve, hilarious personal anecdotes and tributes to famous physicists. Being a prime participant in the debate with Hawking on the other side, he is in a unique position to tell the story. His recounting of the way the physicist Jacob Bekenstein used high-school math to derive the formula for the entropy of black holes is astounding; very rarely has someone used such simple physics and mathematics to discover such profound relationships and the act reminded me of Bell's Theorem, another spectacular twentieth-century physics result that can essentially be derived using high-school mathematics.
But more than anyone else, it is Hawking's figure that looms large in the book. Susskind describes how his physical disability, his strange disembodied computer voice and his astonishingly brilliant and creative mind guarantees the kind of reverence and silence wherever he appears that otherwise only seems to be reserved for the Pope. Susskind vividly describes a typical Q & A session after a Hawking lecture; Hawking's physical condition means that he can compose even a "yes/no" answer only after several minutes, and what's striking is that during such times Susskind has witnessed audiences of thousands maintain stand-still silence with not a whisper spoken for sometimes fifteen minutes while the great man painfully communicates himself. Hawking may be the only living scientist whose presence provokes utter and rapt silence and attention that one would observe only during religious prayer. No wonder Hawking is compared to God by many, a comparison which only makes him uncomfortable. Susskind describes a particular time in a restaurant where a passerby went to his knees and virtually kissed Hawking's feet. Needless to say Hawking was embarrassed and galled.
In any case, we can only hope that Hawking feels better. However in one way we can rest assured; Stephen Hawking's name has been etched in the annals of science forever. That's the power of ideas. Their timelessness assures us that they remain youthful and vibrant, irrespective of the age and condition of their source. But let's all hope Hawking springs back from this illness to his mischievous, witty self.