It seems like you’re talking about viruses that are new to us, unadapted as you’ve said.

I would suggest that the concern is more for these viruses that don’t need to adapt, such as polio. Viruses that humans have been getting all along, but now the melting permafrost is a new, surprise vector. As well, there are viruses that are perhaps unknown to us, but which were not unknown to our ancestors. Such a viruses may be laying dormant in the permafrost, and ready to wipe us out. No adaptation required.