What prevents priest from delivering message to Romeo in mantua?

outbreak of the plague

There was an outbreak of the Plague that some monks got. The priest didn't want to go into that town.

Friar John arrives to tell Friar Laurence that he was unable to deliver the letter to Romeo. His excuse is that some people were afraid he carried the pestilence (the plague) and refused to let him out of a house. Friar Laurence realizes that this destroys his plans, and orders a crowbar so that he can go rescue Juliet from the grave.