So I was staring at some old posts and watching the chat trickle by and I just thought of something that could explain how this WHOLE thing started.
I don't know if it's been mentioned already but maybe radiation is the cause? I always suspected something was wrong because the flesh eaters are never described as decayed corpses, only depicted as mutations. Perhaps it's due to some byproduct from a bomb dropped in major cities around the world? Terrorists? A secret war between governments?
Maybe some worldwide doomsday cult planted a chemical bomb that mutated and changed anyone nearby? We did hear a HUGE explosion in the beginning when it all started. Then suddenly all hell broke loose.
Michael says the ones on the freeway were covered with scars, maybe they were actually blistering sores caused by the chemical? The big one Kalani and Angel saw was covered in zits, we know some are blind and some smell good whereas others can't. Perhaps the mutations are still changing the creatures.
We know that going too far north into "ground zero" makes people sick. High concentration of this stuff (where the explosion went off) could be the answer as to why this is. It could have all been experimental or a secret weapon that hadn't been tested yet that was stolen. The possibilities are endless.