Destructoid has this interesting theory about the original Left 4 Dead survivors. Could they be carriers of the disease, infecting the world as they go? L4D2's safe house has some scribble on it mentioning carriers that spawned the discussion.
http://www.destructoid.com/why-the-left-4-dead-survivors-may-secretly-be-infected-153722.phtml#ext

They could be. In fact, they almost certainly would be. They'd almost certainly have had some blood to blood contact with infected blood. But they're just immune to the affects.
I don't know if it's possible that they've spread the disease somewhat. But given the sheer number infected I doubt it made any difference either way.

I was just amused at one of the closing lines in that piece:
"it'll be like The Jetsons Meet the Flintstones except good and you'll be able to have Francis and Nick be assholes to one each other and oh my god so cool."
The idea of Francis and Nick snarking off at each other would crack me up. I'd love to see that.

I think the new cast are carriers too..

My thoughts exactly.