Having said that, I do agree that really, you would need mass infection throughout the population, then the zombie stage later, in order for this to get really out of hand. Especially if people reanimate automatically on death, and it wasn't known at first. Hospitals might be the first places to get overrun, actually.
I agree. Something with a very, very long incubation period and easy transmission person-to-person could be endemic to the population before anyone even knew it existed. That would create the numbers needed to give the zombie plague a shot.
From "Zombies:A Hunters Guide" by Joseph McCullough, with some paraphrasing...
"As a general rule, around thirty days seems the cutoff point for a corpse to be reanimated...by then, much of the internal tissue of the body and many organs have turned to mush and escaping gases have ruptured the skin. The first to be affected in a hot zone will be the most recently dead. Hospitals usually form ground zero for a zombie outbreak. One moment, everything is business-as-usual; in the next, a man who has just died on the operating table gets to his feet and attacks the surgeon. Within a few minutes, the corpses in the hospital morgue also reanimate. In less than an hour, funeral parlors and crematoriums are also affected.
There is a grim irony to outbreaks commonly starting in hospitals. Not only are they places where death and corpses are common, but they are also generally full of the ill and injured who can offer no resistance to an attacking zombie. Thus, a single zombie can kill numerous people in a short space of time. These victims, now part of the recent dead, will also reanimate and join the rampage. In such a way, the number of zombies can easily multiply in the first few hours."
Now, since we are only seeing the zombiepocalypse from the point-of-view of our group, with only glimpses of the bigger picture provided by Jenner in the CDC, we just don't know how it began, what steps were taken to prevent the outbreak, if it indeed went completely global, who is left alive out there, if the authorities triaged the metropolitan areas and began to fortify smaller towns, what's going on. All we see is what the group knows.