In Chapter 3 (part 3 of 3), Burt talks about the smart zombies. The climbing zombie has a level three security badge from

"That one there, lying there, trapped me in the bathroom. It just waited for me to come out..."

"You see that badge clipped there to his belt? It's a level 3 ID for Radon Labs. Must have been pretty damn smart to get that badge."

I wonder if we're going to see Radon Labs featured in upcoming episodes? I'm willing to bet that they were involved with the outbreak. Angel's theory of the mental hospital being the source of the outbreak is probably off, but remember how the first zombies - the ones on the highway - were described by Michael as being covered in scars? Maybe they were test subjects; experiments gone awry. A containment failure could account for the sudden outbreak. The first of the outbreaks were reported to be in Los Angeles and downtown San Diego. There are major research universities in both of those cities; there would be research labs nearby, and the biotech industry seems to be clustered up around San Francisco and Menlo Park.

Maybe something kind of clandestine project?