Walking Dead question *possible spoilers*

I'm currently playing catchup. I only have about 4 episodes of season 2 left to watch.

I know that at the CDC they were told it was a virus of some kind. In the "Guts" episode they made sure not to get any blood on their skin in order to not get infected. That was pretty much the last time they worried about it. They've just been hacking and slashing away at them with blood splattering everywhere. Shooting them with blood splattering everywhere. They don't seem to be too worried about that anymore. They just worry about getting bitten.

Do they ever really explain how the virus is transferred via bite? I guess they are basically a big open wound, and when they bite someone it transfers to them via the bite wound. But wouldn't there be some instances where infection doesn't occur? For instance, if someone who just turned into a zombie and doesn't really have any open wounds or decaying flesh, how would the bite transfer the virus? Is it in the saliva?

I'm looking a little too deep into this, obviously.