There are a number of bugs that can affect a hive.
A strong hive can usually resist most of the bugs
There is a trade off between managing swarms and minimizing bugs
Managing a hive requires it to be opened, which both enables swarms, and facilitates infestation.
When full brood frames are pulled, and replaced with empty frames you provide habitat to bee pests
With only a single hive, it is difficult to determine whether it is weak or strong, since frames cannot be swapped with
those from stronger hives.