So. Basically, the root of the idea is this:
There is this pathogen (probably a fungus or bacteria). If you have it in your system, and are otherwise healthy, it'll hang out in your brain tissue and/or nervous system, hiding from your immune system and quietly growing. You will have no obvious symptoms, though the pathogen could be found on a sufficiently intensive medical scan. If you die, or are close enough to death to trigger the pathogen, you become a zombie.
Now, if you have one zombie, you have a mindless shambling thing that can be stopped by a closed door, or sometimes a well-placed wall. Think "will try to walk towards the nearest food, regardless of obstacles". It will chew on things that it happens to catch, but if you are paying attention at *all*, you can escape pretty easily.
If you have 10 or so zombies, you have... a perfectly-coordinated pack predator, about as smart as a not-too-bright dog. Fairly dangerous, especially since if it bites someone, then kills them, it gets *smarter*.
If you have 100 or so zombies... you have something as smart as a person. And it gets smarter, as you add more zombies. Now, it's not necessarily malevolent. It is self-interested, and it's not in its self interest to kill all humans. It needs us, in order to get more bodies, if nothing else.
The pathogen can only spread by fluid contact (blood, sex, bites, possibly kissing). It can't survive outside of a human body or adequately near equivalent. It can infect someone up to maybe a few minutes after death (when the body's warm and still has some fluids gushing around).
An individual zombie body can be killed by sufficient head trauma (they essentially hijack the neural architecture of the host body), and it will also simply wear out if left alone, after a few months if not fed/tended, or a few decades max if properly cared for. It needs high protein food (preferably human or animal flesh). And it can also be slowed or killed by various environmental extremes (vacuum, extreme heat, extreme cold), though it is less sensitive to those than a human would be. It does need oxygen, but only a small fraction of what a human would need.
The way I pictured the original outbreak:
Patient Zero had a fungal or bacterial brain infection that mutated into the zombie pathogen. She died, then proceeded to infect several people before she was stopped. Nobody knew what was going on at this point, they didn't really realize she was a zombie, they just thought she was dangerously insane or something.
The infected people (not realizing they were infected) proceeded to infect a few dozen others (through sex, blood donations, et cetera) When one of them was run over by a car, another nucleus of infection was started, and authorities started to get a clue that Something was up. But they hadn't specifically isolated the cause or anything.
Then, one of them died in a large plane crash, along with a hundred or so other people (most dying of smoke inhalation or internal bleeding or the like, rather than gross trauma). The zombie, who was one of the first to die, started to wander around munching on the other dead people (stopping once they were infected, and thus part of it). Pretty soon, the first true zombie mind was formed. It was smart enough to realize that it needed to figure humans out before doing anything, so it disappeared into the wilderness, to give it some time.
Using the available resources (probably including at least one internet-capable smart phone), it studied humans. Then, to make it so that one military strike couldn't wipe out most of the zombie threat, the zombie mind deliberately introduced the pathogen into some unprotected part of the medical supply chain (maybe flu vaccines, or stored blood, or something). Possibly several sources--illegal injected steroids, maybe infecting and killing a doctor, then having that doctor give "vaccinations" to all his patients--basically, enough people that in order to wipe out everyone who was infected, they'd have to kill an unacceptably large fraction of the population. Then, they opened negotiations...
But, I'm not quite sure where to go from here.
If you have any thoughts, about either the idea or what to *do* with the idea, please comment...