Nucs and packages (and migratory colonies) introduce pests and diseases. Routine drift disperses them. There is very little chance that any colonies are ever isolated from these problems. Just look at the rapid spread of Varroa destructor across the country
and into every colony in existence in a very short time after it was introduced.
In the cases of mites or small hive beetles, yes, a reasonably healthy hive with the ability to manage the problem (i.e. mite resistance or timely and effective treatment) will usually be ok unless the pest levels are simply overwhelming.
However, with viruses in particular, a reasonably healthy hive may never have been exposed to a virus or virus variant that gets trafficked into our region, and high mortality can occur. The recent spread of VDV-1 virus is such an example. Higher than normal
mortality rates over a given time period are often related to the introduction and dispersal of these kinds of agents. Once a new virus gets into a colony, even if by Varroa mites, it can often be passed from bee-to-bee directly and cause harm even if mites
are kept at low levels.
Joe