I see some HAs with Morostide Human Zapper skins yet they still manage to have become zombies this year or have cured and are zapper avatars once again. I thought HAs with zapper skins could not get infected, so how is this possible?

Did they have the Baker's Kiss? People with Morostide zapper skins were able to be infected with the Baker's Kiss so they could participate in the plot.

That explains part of it, but I saw another who had the Rose disease (which they concealed until they cred themselves, and they were revealed to be a Zapper avatar as well.

It was a glitch at the beginning of the plot

Well it sure was a nice "glitch" and one that did not get fixed. It worked for the plot, at least.

here is the topic if you wanted to see it.

Thanks