Random Ramblings

You're not over-thinking it at all! They never specifically said, but just from context they give you the impression that one dose of anti-virus cancels out one dose of virus, in essence. Carlos got bitten once in Apocalypse, and Alice gave him a shot of antivirus and it stopped him from getting sick *that* time, but it's not a vaccination, because Angie had been given the virus on a massive level and she had to take antivirus regularly, all the time, to keep from becoming infected. That's why she carried it around with her in that lunchbox. So the shot Alice gave him in Apocalypse cancelled out the bite from Apocalypse, but he got bit again in Extinction and would have needed another shot of antivirus if he was going to survive. But you only have a certain window in which to administer the antivirus before it's too late to stop the infection. That's why Alice asked him how long ago he was bitten when they met in Apocalypse and why Carlos told her in Extinction that it was "too late" for the antivirus. (Which is CRAAAAAAP, but only because I really, really, REALLY wanted him to survive and then he and Alice could have repopulated the Earth and after that *everyone* in the world would be beautiful! Sigh.)

At any rate, the only exception to all of this is Alice. The virus bonded with her DNA and, basically, became a part of her, causing her to mutate, so she can't be infected. She's the only person in the world that's immune. That we know of. And that's why she has super powers.

But the antivirus SHOULD work the way you described above, just because then Carlos would still be alive :)