What does pestilence mean as it is used in the following lines from Act V, Scene 2 of Romeo and Juliet?

Friar John: Going to find a barefoot brother out,
One of our order, to associate me,
Here in this city visiting the sick,
And finding him, the searchers of the town,
Suspecting that we both were in a house Where the infectious pestilence did reign,
Seal’d up the doors, and would not let us forth;
So that my speed to Mantua there was stay’d.

In this context, pestilence refers to a highly contagious and deadly disease or epidemic. Friar John and another brother were suspected of being in a house where a contagious disease was present, so the searchers sealed the doors to prevent the spread of the sickness. This prevented Friar John from delivering an important message to Romeo in Mantua.