Bab 172: New Source

Ever since he became a Level Two° human, he still hasn't asked the System about a Level Two° human's limit. 

A normal human can have a maximum of 4.9 as their stats. Entering the 5.0 stats means they have already surpassed the human's barrier and become a Level One°. A Level One° human can then raise their stats to 14.9 by consuming and absorbing Apocalypse Viruses from the zombie's heads.

A Level One° human means their Will or the guard for their brain had already become stronger. It would already be hard for the zombies to infect them. 

Entering Level One° means a human already has an Apocalypse Virus inside them, the only difference is that in the zombie's case, the apocalypse virus is the one controlling the human body while on the other hand, humans use the apocalypse virus as a tonic to increase their strength.