Good points guys.

Perhaps the incapacitation was due to hypothermia plus another factor like altitude illness or hypoglycemia. Just as widget describes, the altitude factor can whack you seemingly out of nowhere. I recall getting really whacked at only 8,000'... then two days later hiked up to 12,800' like I had wings on my feet.

I'm not sure I ever felt worse than when I had altitude illness. If in that condition I had to climb onward, I don't know that I could have done it. I didn't even feel like I could make it across the campsite.