In Philosopher's Stone, chapter fifteen, The Forbidden Forest, while serving detention, Harry, Malfoy, and Fang encounter a robed figure, who ultimately turns out to be Quirrell, drinking unicorn's blood in the forest. Harry later learns Quirrell was drinking unicorn blood to sustain Voldemort's life while Quirrell was serving as a partial Horcrux for Voldemort. Canon states that anyone who drinks unicorn blood will be granted life:
‘That is because it is a monstrous thing, to slay a unicorn,’ said Firenze. ‘Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenceless to save yourself and you will have but a half life, a cursed life, from the moment the blood touches your lips.’
Philosopher's Stone - page 188 - Bloomsbury - chapter 15, The Forbidden Forest
Why did Quirrell, who had infused himself with unicorn's blood regularly, die when Harry Potter touched him, while Voldemort's spirit surivived? I suspect the answer to the second part of the question is because Voldemort's Horcruxes protected him from death. But why wasn't Quirrell afforded the same benefit via the unicorn's blood? Despite consuming unicorn blood, Quirrell died and Voldemort didn't. Why?
Please no HP Wikia/Wikipedia answers. I'm looking for a canon-based answer or a J.K. Rowling quote.