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ScienceJournalist said: It's become a common zombie trope that the extra bacteria and other parasites (doesn't have to be bacteria) living in a zombie's saliva accelerates death so that the virus/prion/run-amok stem cells can take effect. This doesn't really explain a scratch unless the parasites can be transferred via plasma leaking through cuticles (which it is want to do in decaying bodies).
As for exposure limits (why Daryl can fellate recycled arrows), you'd have to get a decent dose of said parasites or "bugs" for them to get past first line immune defense mechanisms like acidity and mucosal layers--and there's a good chance that the bugs wouldn't live in large enough numbers of for very long outside the pestilence-nurturing goo bag that is a zombie's body.
But what the hell do I know...this laying logic on the same people that gave us the kick ass but utterly stupid zombie car.
As for Rick--he's pissed (and correct me if I'm wrong) because Lori was riding renegade cowgirl before the zombie apocalypse ever went down, and the kid might not be his. That sort of thing leaves a sting for a while.