I think this is a pretty good answer to how a ZA could happen. Basically, as outsiders looking in, it's easy to say we'd just immediately realize what's going on and blast out zombie brains.
But in a world with no Romero et al, or even in this world if this actually started happening, I'm not convinced people would be immediately comfortable with blowing other people's brains out - even if those people are lumbering towards them looking mighty hungry.
How long does it take before you decide that these walking, moaning humanoid corpse thingies are something you can just kill. How long until you decide there is no hope for a cure, so locking them in a wing of a hospital does you no good. And not everyone will agree on this. So *you* may think it's okay to kill them, but does your next door neighbor? Bring on the arguing. By several months in, I'm sure more people would be in the blow their brains out camp. But one day, a week, or a few weeks in? With I think 30ish% of Americans opposing the death penalty?
And then there is the whole people do dumb things issue. I'm bit? Oh *I* won't turn... I'll be the exception. I just won't tell anyone... It'll be okay... Honestly, if people are selfish enough to scam senior citizens out of their pensions, they are selfish enough to cause a lot of havoc during a ZA.
Add to that the whole how long it takes to figure out that whatever is happening spreads through bites and can only be stopped by destroying the brain (not generally a common target - from what I've been told most people are trained to aim for the chest since it's an easier target), and you have a recipe for chaos. Then if the actual re-animation time varies it would be even harder to trace.
So imagine, you have Patient 0, who stumbles "drunkenly" into a bar moaning (after being bitten by a radioactive spider from the planet Krypton who had been caught in a transporter beam and ended up spliced with a platypus or something - but don't worry he squished the spider after it bit him). Bartender says, "I can't serve you". Patient 0 moans again, staggers forward and starts grabbing at some lady at the bar.
Lady screams and tries to shove him away, punching him in the mouth, and nicking her knuckle on his nashing teeth. Always ready for a brawl, bar patrons start pulling the man off, and as the brawl gets out of hand (the guy just won't be knocked out afterall, no matter how many punches), the police are called. Some people have sustained scratches or bites - who knows if it's from Patient 0, a random patron, or a random patron already bitten by Patient 0.
Police try to arrest the guy, one of them gets bit in the arm, but not badly. The guy seems drunk and out of it, so they take him to the hospital, thinking he's on something. Because who would suspect he is actually dead/un-dead?
So Patient 0 is restrained at the hospital. Meanwhile, bitten Lady gives a statement to the police and goes home. Maybe some bar patrons go to the ER for stitches, maybe they just go home. It's not the first time they've been in a brawl. Several of them wake up "hungover" the next day. Man, they've never been this sick.
Should lay off the sauce. Maybe some go to the doctor. Maybe they stay home. But unless they all live alone, at least some of them are near new victims. Some probably get sick enough to go to the hospital, but some probably just die at home - especially depending on just how fast this thing spreads. Guy bites his wife, wife freaks out and runs out of the house, he's been drunk before, but never attacked her like that! Wife goes to stay with her sister. Yum. New victims.
Meanwhile, Patient 0 is at the hospital, and everyone is confused. Does he have a heartbeat? Brain patterns? Well this is fiction, so we can say what we'd like. But whatever is going on, I'm sure more and more experts are being called.
Maybe some suspicious information has been reported by the news by now. But certainly nothing concrete. That wife that went to stay with her sister? Yeah, she's not feeling so well now... Sister thinks she's just upset about her husband. Next thing you know sister's kids are zombie chow. Sister comes home from work and flips out finding blood and a badly bitten and bleeding child. She immediately calls 911. She can't find the wife, doesn't know what happened and is hysterical.
Perhaps a few more "patients" have ended up in the hospital by now, but still, now one is really sure what's going on. Maybe someone has started to notice they all have bite marks, but if some are in less noticeable places, it could take awhile. Plus, since human bites aren't necessarily a well studied disease vector, it may not jump to mind immediately. Bottom line: more confusion.
Eventually, someone will connect the bites to the fever to the reanimation. But by this point, I think there could pretty easily be hundreds of zombies, especially dependent on how fast the fever works and reanimation times. If any of those caught a bus or plane out of town, you are in even more trouble.
But now, the battle is only just begun - how do you stop these things? They don't seem to respond to pain. You can shoot them in the heart, and they just keep going. Meanwhile, should you kill them? What if there is a cure? Doctors aren't generally in the business of killing their patients. So Patient 0 could just be there for awhile. Along with other people showing up with bite wounds. Maybe they come up with some place to hold them, kind of like a quarantine, while they work on a cure. Fantastic. Until whatever quarantine security they have fails. Now there is a *mass* of zombies loose at the hospital. Meanwhile, there are those that never made it to the hospital wandering around causing problems.
Eventually, we call in the military. But how long before the president gives the kill order? How long before he authorizing shooting American citizens in the head. Without a trial. Without them necessarily doing anything wrong? Can you imagine the flack he would take if he was wrong? (Of course the president is a he, because this is a zombie scenario, and only men are leaders in a zombie scenario) So while congress is arguing, the zombies are gaining ground.
My guess is that in a real world scenario, there would be a pretty thin line between the threat being nipped in the bud with only a few towns/cities being affected and an all out outbreak.
But that level of confusion and the ethical struggle and the politics aren't an easy thing to portray in an entertaining way - hence why they are generally skipped :-)