If there are dirty frames that are contaminated with mold or feces after overwintering, they should be replaced with new clean built wax combs. Following the number of bees and looking for abnormalities in bees, brood, smell etc. during every inspection gives you continual information about the bee colony’s health. Observing the flight entrance and the behavior of the worker bees tells you the bees’ foraging ability and harvest at the time.
To avoid spreading infectious diseases to a new nucleus, you should ensure that your colonies are healthy. This can be done with either a visual inspection or by sampling the bee hives for a laboratory test (for instance, every fourth year). Swarming is a natural way for the bee colony to get rid of microbes. The swarms can be placed onto a new wax foundation once the colony has completely renewed its comb structure and diminished the infection pressure.