Well, first off is the magic of TV But also, everyone is infected already, the bite isn't necessarily making you infected, but it is causing symptoms (fever etc) that you don't recover from, and then you die, and the virus which you already have in you, brings you back. So it's the fact that you die that turns you and not the bite. And you'd think that all that juice he covers himself with would cause you to contract the fever, but I guess this is where tv comes into play.

 

Easy. It depends on how the 'virus' [or whatever the pathogen is] works. You can have HIV infected blood splashed on your skin and not become HIV+ as long as it doesn't come into contact with an open cut or sore. And while HIV infected fluids entering your mouth is a risk, due to mucous membranes, you actually still may very well not contract the virus, especially if you have no cuts or sores.

And if the zombie pathogen works more like a certain snake venom, then there's even less chance it would effect you through ingestion. Some snake venom can be ingested without any harmful effects.

 

Nick could be immune, due to being in a drug induced, semi-comatose state ( like Rick ) when the pathogen / agent was first introduced. Neither Rick, nor Nick ( coincidence ? ) caught the cold / infection ...and so are now walking reservoirs of antibodies that can fight the weaker, or mutated, strain. Both of their near death like conditions prevented the initial, hidden outbreak from taking hold .