In my opinion, in all reality the only thing that is going to make the dead seem to come to life is a mutation of the rabies virus. L4D is a prime example, that's what makes human so savage in that game,..soo i'd assume a huge possibility!
(mind the mutations which could or couldn't occur)
I'm not a fan of the zombies with special powers...I mean if you couldn't do it when you were a regular human, you won't be able to do it as a zombie. Don't get me wrong, I love the game, I just don't think that's very "realistic."
There is no way one virus would realistically possess the ability to simultaneously transform these people into what they are based on what they do.... in fact LZ's explanation of why some zombies are fast is the only one that has ever made sense to me, and runners aren't even "Special" in L4D.
The infected in L4D are just that - "infected". They will never say "zombies", "ghouls", "undead". This is also why body shots will kill. As for the realism of it, I just don't know. I believe the Campion Virus is a plausible mutation of Flu and/or Rabies.
To answer your question correctly, Yes.
But not "yes" as in, "Yes, that's how the zombie invasion will pan out", but "Yes" as in, "Well, yes, this is as real as it would be."
Now examining HOW the term "Zombie" applies itself to Left 4 Dead...
To be correct, no, the infected in Left 4 Dead are NOT zombies. They are NOT undead. They did not fall down(dead) and get back up. At least we haven't seen it yet. They are mutants of a sort which bare similarities to zombies in that:
1. They kill (mainly) humans.
2. They bite to infect humans. (although we never see this it's implied.)
3. They were once civilized (hinted by their clothes) but something got inside of them and turned them into violent pricks.
4. Their main purpose seems to be to feed or infect humans.
5. Headshots are (sweet) instant kills.
But the diffrences are very noticeable:
1. Headshots are NOT required to put them down.
2. They now how to navigate areas run, jump, and climb.
3. They express awareness rather than just being driven completely by instinct.
a. Finding walls to lean on. (They seem to get bored)
b. Fighting with each other to the death should they get in each others way. (They seem to be territorial)
c. They throw up while holding their stomachs and writh when you shoot them (non point-blank/hunting rifle) while they are unaware of you. (They seem to feel pain.)
d. They make sounds and noises that hint at anger and sometimes sadness. (They seem to have a slight sense of emotion((Is it me, or did one of them say gargle "mother"?)))
Feel free to correct me or list more differences because there are a HELL OF A LOT of varying facts.
So to FINALLY answer your question. If ever, a Zombie Invasion. This is probably the closest we could possibly get to one, a "Mutant" Invasion. Of course, there are certain things about Left 4 Dead that realistically wouldn't make the final cut for reality.
1. People who are actually immune.
2. Special infected.
3. Pipe-bombs, molotovs, guns, ammo, etc lying everywhere lol.
But needless to say, unles the "virus"(or whatever this thing would be) is also airborn. I think the government would be kinda quick to contain it so um...yeah.
Though, that's just some of what I think. Most of it's all just hypothetical horseshit. So grain-of-salt time, everyone!