Crusader Kings II The Reaper's Due DLC

Once prosperous provinces were devastated, and the master of the country hides, retreating to his fortress at the top of the hill, hoping that the plague will pass him.
The wiser princes took precautions and did not squander the income of the more prosperous years. But in time, they all have to pay for the Harvest.

The Reaper's Due adds a series of new mechanics centered on the Black Death and other epidemics that shape the world.
Close your gates to prevent the plague from infecting your yard, even if it means isolating yourself from suffering citizens.
Peace and good management will make your provinces reward you with more wealth and manpower, but constant war and disease can depopulate your lands.