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.