Put yourself in the shoes of a feudal lord (circa the year 1546) for a second.
Your country is experiencing small outbreaks of plague. The few counties under your control have been spared, but three years ago you had a devastating disease ravage your livestock and kill off a third of the peasants under your rule. Some members of the populace are asking questions. It seems that letters that were sent off months ago have not been responded to as of yet, and they are getting worried about relatives living in not-so-distant areas elsewhere in the country. You recognize the name of the county as one hit hardest by the plague outbreak.
Of course, you know exactly why there hasn't been contact from those counties. You (being the wise ruler you are!) set up a cordon sanitaire weeks ago.
A cordon sanitaire is a line of troops posted on the frontier of a county (or country, in some cases) in order to prevent communication of the inhabitants with those of a neighbouring country in which a pestilence disease prevails.
Why would you want to do this? Foremost, to prevent the disease from reaching your front step. A few soldiers dying on your borders is relatively a benign problem, and doesn't raise too many suspicions from your peasants. Which leads to the second reason for a cordon sanitaire: crowd control. Nothing spells disaster like all of your county's residents packing up and leaving hastily because death has moved in next door.
So next time your local militia goes missing for a few days, your mail service is interrupted for a few days, and your local politicians are all smiles and whispers, consider that vacation you've had your eyes on soon. Just don't go and visit Aunt Mildred down the highway.