The pathogen being released through fissures doesn't really jive with the order of events though, despite the incident of someone turning just from being there...
The infection started all over the globe nearly simultaneously. That seems unlikely for mother nature to pull off, at least without some help. Also, these fissures also seem to spur "evolution" of the zeds. These new species make zeds much more effective, especially when you factor in their appearance coincides with a certain level of societal degredation.

Regular/Smart ones - mass confusion and panic, high number of uninfected concentrated in small areas. No one knows what is going on, easily taken by surprise. Smart ones organize/lead to make the process more efficient.

Runners/Jumpers - uninfected have thinnned out considerably. Starting to arm themselves, building defensive positions, using cars/transport much more effectively. Zeds need to be faster/more agile to catch more dangerous prey.

Behemoths show up/jumpers die out - uninfected have become heavily entrenched, small arms incredibly common among survivors. Zeds need strength and durability to oeverpower their prey. Jumpers, while agile, lack speed and can be gunned down easier, causing their demise.

Little Ones - Survivors are heavily fortified, gaining access to heavier weapons, building a self-sustaining environment/less likely to leave fortifications. Need a smart/fast/strong/durable invading force to break defenses, allowing other zeds to infiltrate. Think Zed SWAT team.

The real outlier here is TOWTM and possibly randy. TOWTM is not only smart, it also has the abilities of a jumper/runner and seems quite durable (i think he's wearing a flak jacket correct?).