Why is it so hard for the characters in the manga to realize what's is going on!?

People are biting each others faces off, and yet they try to help them....

the only possible answer to that is they are very very stupid...thats all there is to it and would you really believe that someone was a zombie in reality? i doubt it...so another factor is denial of whats ocurring

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I don't need to believe in zombies to know that somebody is trying to eat my face; either way I'd make use of the shotgun.