How can you prevent the spread of the black death or plague the next time it hits?

Depends: Enough cats might have controlled the rat-borne bubonic plague, but the next worldwide outbreak of deadly disease will probably be a droplet-borne respiratory illness. Until enough immune serum can be harvested, the precautions used in the recent SARS outbreak may serve as a model. And you do a good thing in asking -- because it will happen sooner or later.

Unlikely: There's won't be another plague epidemic unless there's a bioterrorist attack. The big difference between the modern era and the middle ages is that we now have antibiotics that treat yersinia pestis, the cause of plague.