The general belief of the surviving populace in the apocalypse concerning the monsters they encounter was that monsters were the negative by-product of the mutation that occurred on Earth.
When the apocalypse descended, it came along with cracks in the sky, and at that moment, every human on Earth experienced something similar: a burning sensation coursing through their body.
However, while the sensation was initially the same for everyone, it later branched out in three ways.
One, which accounted for the majority of the humans on Earth, involved releasing blue gas after the initial hit sensation. Most humans in this category remained largely the same as they were before the apocalypse.
The other group started the same as several humans who did not release the blue gas, like others ended up collapsing from the burning sensation. These people mutated, with one side having a positive outcome and the other with the opposite.