They seem to forget to mention "Dawn of the Dead" where they gave them a more "realistic" point of view where senses and speed were there. Zombies are a creation of only that of a highly mutated form of Rabies spread onto humans.

There are diseases/Viruses that if you get, easily within 12-20 hours your body can literally eat itself to the looks of that of a zombie. I find that Zombies are a possibility and depending upon where it starts and how it is taken care of it can spread. It takes one bite, that person walking away, "As they show in the movies" and then infecting an unsuspecting neighborhood who they themselves spread.

1 > 2 > 4 > 8 > 16 > 32 > 64 > 128 > 256 > 512 > 1024 > 2048 > 4096 > 8192 > and so on and so on, with this being so of less than 20 hours per infection, unless taken care of quickly it can spread within 7 days within a city.

It's like the flu and how would you know that it starts out as one person, likeliness, if one gets it there are hundreds or thousands of others nearby that are experiencing the same problems within weeks time without originators spread.