Zombies don't look like shambling corpses all of the time. Usually they look like- and act like- living people. When they succumb to zombiism they do not lose memories, skills, or any other aspect of their original personhood but everything becomes subjected to two overriding drives: to eat people when hungry, and to stick as close as possible to the life that one had before becoming a zombie. Zombies don't change their habits at all except so far as is necessary to cover up their cannibalism and carry it out more effectively. They also don't eat very often: a single person has enough meat to keep a zombie looking fresh (they start to turn corpse-like over the course of a month if unfed, and also look slightly corpse-like when they will be feeding soon or having recently fed) for a week, or maybe two weeks if that person is only eaten over time and is a little bit overweight (this requires cutting them up while alive, to keep them from turning into zombies themselves). Yes, this does mean that some zombies kidnap people and fatten them up to get more meat out of them.
Human settlements become infested with zombies usually when a zombie comes into town. Someone eventually escapes but before getting eaten, and becomes a zombie. The zombie sticks to its original life as much as possible, even refusing to leave the town (unless travel is a part of its normal life; truckers-turned-zombies are really, really bad), and new zombies are created from people who escape being eaten but not being bitten, and sometimes when the zombie decides to recruit some people in order to make hunting easier.
Eventually, either the zombies are killed or the town is eaten, which leads to the zombies slowly growing hungrier, corpsier, and crazier until the drive to eat overcomes the drive to stay normal and they wander from the town, becoming your normal corpsey-zombie (but a little bit faster and definitely a bit smarter). They'll eventually recover their full memories and freshness unless killed before they get a good meal, but now they don't care about sticking in one place and they just wander around, essentially nomadic serial killers. Nomadic zombies are responsible for most outbreaks, and usually have left the area before a containment team is notified and sent to the area, so they're not only crazy but also often very old.
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