lazy writing
they don't talk about how the little girl might also be infected = nothing happens
they mention it once = five seconds later she bites her mom's neck out
also, after they learned how dangerous the infected could be, why do they keep trying to engage them in physical contact? like, you just saw a girl bite her mom's throat like at least five other people had, and now you're waving your hand in front of her face? also, stop turning your backs to the infected. like, cool, you separated yourself from an infected dude behind a glass door. maybe don't stand with your back right against the glass.
finally, after they discovered an effective way to kill the infected, why didn't they just keep doing it? just hold on to that sledgehammer, try to bait the infected toward you one at a time, and then crush their skulls. just because you did it once doesn't mean you can never use that same means of killing again.
but lol at the old lady just standing there, in full zombie mode, totally not displaying any of the rage and increased physical abilities of any of the other infected people