Consider this:
In order for anything to become immune to something, it has to be exposed to it. For example: we'll call the first type of cold, Cold A. If Cold A is caught and spread by person A to person B, C, D, and E. Then they overcome Cold A. Cold A must then mutate to be able to reinfect Person A, B, C, D, E. However, this is only the case that our immune systems adapt.
If, for any reason whatsoever, our systems did not adjust to the version Cold A, then Cold A would never turn into Cold B, C, D, and E. Any exposure to a bacteria, or virus is such like this. The only one exception I am awre of is the HIV (AIDS), and all of the other viral STDs.
However, in the case of the Guyver Virus, the virus itself (For the sake of argument, we'll call it GV) may adapt to the Guyver as well as the Guyver will adapt to the virus, but IF the GV were to adapt completely, then it not only will become lethal to the Guyver, but also possibly to the carrier. This would mean that not only would Sho die, but Tetsuro, and any other HUMAN BEING or organic thing would die as well. Not only would Chronos have to beable to cure the first wave of the Virus, but they would also have to very quickly learn how to quickly develop an antidote for any other form. This would mean that not the viral outbreak would eventually become an epidemic if it was not properly released, placed into the desired target, etc.
MEANING: It would eventually kill everything, even the Zoalords.