I have read every single issue of the comics 1-96. Nothing there as to cause. Kirkman said in an interview that he has some ideas as to the cause but that he will not ever release them in comic or TV. He feels the cause is irrelevant.
So we do not have, nor will we have, enough data to conclude whether or not there are multiple viruses at work or if it is one virus with a dormant/active state or one virus with a catalyst mutation factor.+Preston Osborn
, I think that your use if blanket generalities and stating conjecture as fact is only confusing the discussion.