Firstly, discuss historical events regularly with your children. Then you can have conversations like this one:
Son: So, does the black death still exist?
Mum: No, the plague was carried by rats, and transmitted by fleas. Fleas would bite the rats then bite humans, giving them the plague.
Son: Do we have fleas in our house?
Mum: No of course not, we live in an age that has embraced hygiene.
Son: Only dirty places have fleas? Like my room?
Mum: Of course your room doesn — why, yes, fleas could show up in such a messy room.
Son rushes to clean up his room. And my work is done, for a few days at least.