How oftin the Plague/Black Death came back.

The Plague came back about every 20 years.  As the humans began to realize this disease was contagious they thought there was only one way to stop it.  So the humans decided they had to get rid of the victims.  So they built a wall around the Italian city of Sanserolero.  What they didn't realize was that the black rats were the ones making the Black Death contagious.  So all they really had to do was kill the rats... well also the fleas.  So after a while they realized people were still getting sick.