That plot description isn't very good, as far as I can tell there was no "virus" - even as a mystery, it wasn't referred to as viral.
As for the manga ending, what a shame! That being said, it is very good, and I would rather it finished properly. It seems the mangaka was tying up most of the loose ends anyway, so dragging it on would make little sense. I just hope it was ended properly.
And I do hope that it does get an anime adaptation, as I think it could be good. I'm not particularly fussed about Gonzo and AIC, I feel they wouldn't be able to do it justice. If IG or Madhouse got their hands on it, that would be swell.
Alive is pretty underrated, but it's become a real gem.