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Don't know a lot about The Walking Dead, but I am interested in zombies. For the spread thing; it could involve a kind of viral agent and there might be people who have a specific physical trait or mutation that grants them an immunity. So maybe the infected fluids not only have to come into contact with the bloodstream and with someone who is not immune to the virus, could also explain why some characters get infected over time; the stress of living in zombie world breaks down their immune system making them much more suseptable to the virus?
Maybe the zombies that lay in wait for their prey have learned the behaviours through conditioning. Say that one zombie for whatever reason was just laying there, maybe it was missing its legs or something and not moving, if a person walked by the zombie and it was able to attack them it might learn through conditioning that laying still and looking dead might be a way to get food with out having to chase food. Other zombies might learn that laying still means easier access to food by seeing how the smart faking death zombies do it. I know it seems like a lot for zombies to learn, since they're rotting corpses; but a lot of conditioning experiments have been done on things like flatworms; so could be possible that a really fresh zombie could figure this out with enough repititions.