Bingo.
I'd find the other ones.
Although the original question makes an interesting thought experiment (and certainly was only intended as such), like most "lifeboat questions," there's a lot more to it if it were ever to become a real situation.
I can think of nothing at all that would kill almost every person on Earth that would leave the planet with anything remotely resembling a livable biosphere. Even in "I Am Legend," is was stipulated that millions survived the plague, as they would with even the deadliest virus. The hero of the story dismissed this as impossible because he was deeply depressed, and crushed by guilt. Not because it wasn't true.
"Last people on a Mars colony" might be plausible.