[dredmorbius]

Mechanism and agents of infection weren't understood at the time of the Black Death. We know what causes Ebola, and how it is transmitted. We know how to selectively allow what we need for survival (water, food, other commodities) to transit between infected and noninfected zones.

No, not every individual knows or understands this, particularly in primitive areas. But the global system as a whole does.

As for the Black Death, there are instances of specific villages or households (or castle-holds) who sealed themselves off and had sufficient stores to see themselves through the plague.