This is really funny to me because once again I'm being different from everyone else watching this movie, which I must have seen about a hundred bajillion times. And the answer I came up with for the ending is:

I simply don't care.

I have never concerned myself with who is or is not a thing at the end of the movie, and I'm okay sitting with that uncertainty. Apparently, I'm one of five people who is okay sitting with uncertain endings (like Inception). I just sit with whatever the storyteller gives me at the end, and just assumed that some other point was being made, rather than concerning myself with what my nephew likes to call "side-quests".

I have never cared about the answer at the end, and never concerned myself, in any particular fashion, with who infected whom during the movie, either. I feel like I'm one of the few people who feel a great sense of exhaustion/relief that Childs' and McCready's ordeals are finally over, just like them.