well on xbox you can't, you have to be online but im not sure about PC
LOL jus use a cheat code
nope
Single player?
no
No, you can only play as infected in multiplayer.
Yes u can just not the Multiplayer
no
You can't, the survivors you play as are immune to the disease the infected carry. So it is impossible.
To play as a zombie, you must play on the infected team on Versus mode.
1st of all it's left 4!!!!! Dead and you have to play versus mode in the first. in l4d2, it is versus and scavenge.
if u have live play vs otherwise there isn't a way other than modding to play as infected
I believe playing as an Infected is only available in Versus Mode