Even HIV has a few humans that are immune to it, naturally. Bubonic Plague wasn't 100% deadly, either; some humans got it and actually lived. There are so many of us that even if only a fraction of a fraction of a percent didn't die from it...we'd still have enough to start over. Maybe only a few dozen humans, but that's enough.
So it's wildly unlikely. Wildly, wildly, wildly unlikely. It's not entirely out of the realm of possibility, but it's way close.
Something wiped out almost all the cheetahs several thousand years ago, and kept their numbers so small that all modern cheetahs are essentially clones. You could do an organ transplant between any cheetah with no worries of rejection. You might get humans to almost the same point...but we'd make it. ;-)
And that's assuming that we don't find a cure to it first! Worst case scenario, I could see a few dozen isolated scientists searching for a cure in a germ-proof facility.
And we only need ONE survivor, after all. One female, who can get into the frozen sperm stores...and we'd restart the whole thing. ;)