I don't know if someone's already put this idea forward but I think that the origins of the zombie apocalypse have something to do with the Psych hospital and the murderer named ink who escaped. I believe whoever was working at that facility was experimenting on patients (electroshock treatment, torturing patients, or injections of somekind).
This might explain the scarring on the initial "zombies" Michael saw on the freeway in the first chapter. If Ink (the zombie) was able to resist electroshock it would explain why he was smarter than the others.
On the We're Not Dead podcast, they discussed this but couldn't figure out how the people from the tower would figure any of this out.
I think they will at some point return to the hospital, for further exploration, and find a journal of one of the scientists/ psychiatrists/ psychologists, ect. Don't these kinds of people always keep notes?