Friar Laurence's letter to Romeo was intercepted by Friar John, who was unable to deliver it to Mantua due to a quarantine imposed on the city because of an outbreak of plague.
Did Friar John go Mantua himself or he try get a messenger?
Friar John did not go to Mantua himself because of the quarantine. He tried to find a messenger to take the letter but could not find anyone willing to risk entering the city.