At the time the rules weren't established. They could've been killed by pies to the face like they originally had them do in the commentary. It could be in the air or as the tagline says when there is no more room in hell the dead shall walk the earth. Some could be hidden by clothing.

Actually, the rules were established in the film itself. This is deduced from the opening TV show scene and the projects raid. Also when Roger gets bit, he dies and becomes one. Sure, some of them could have been hiding their bites under their clothing but its quite a stretch to say that so many were.

@boxcat said:

My answer shouldn't be your best bet but i guess their flesh has now been rotten as they have been the undead for long. May be a scratch by nails could have turned one in.

Zombie infections and their spreading methods have all been confusing with so many undead films out there. Let's take for instance TWD, where one just dies naturally and turns into one. Then we got ROTLD and this movie follows a totally different concept.

I guess this is the best one i could come up with.

True. None of them follow the same rules and nor should they. Each series should have their own set of rules for continuity. It just seems this film just completely ignores its own rules.