As the Left 4 Dead 2 demo recently illuminated, some survivors in Valve's zombie-filled world are actually "carriers," people who can transmit the supposed zombie virus but are immune to the infection itself.

After a full five minutes of research, I have come to the conclusion that Bill, Zoey, Francis and Louis may, in fact, fall under this category. As much as we all love these four survivors, is it possible that with every step toward safety, they are unknowingly spreading the infection further?

Though nothing is certain and I fully hope that I am wrong, more evidence points toward the original survivors as infection carriers than I would really like to admit. By blasting their way through an infected wasteland, have the fearsome four actually made the world worse?

Hit the jump for our analysis.

The survivors are not, and cannot possibly become, zombies

Though we don't know for certain whether Left 4 Dead's zombie virus is airborne or transmitted by physical contact, we do know one thing: certain people, for whatever reason, are immune to it. The