The whole quotation is "I could not send it (here it is again) nor get a messenger to bring it thee, so fearful were they of infection." The scoop is that Friar John is suspected of having the plague and so has been put in quarantine. So strict is this quarantine (because they are so fearful of spreading the infection) that the letter itself could not have been taken out of the room to give to someone else to take it back to Friar Lawrence. That's why he couldn't "get a messenger to bring it" to him.
Friar John got sent back to Verona because of the quarantine he encountered on the way to Mantua.
He sends Friar John as a messenger.
Quarantine.
His messenger, Friar John, was quarantined.
================================================================================================================================================================ ANSWER: In Verona, Friar John tells Friar Lawrence that he was unable to deliver his letter to Romeo due to an outbreak of an infection. =================================================================================================================================================================
Friar John got sent back to Verona because of the quarantine he encountered on the way to Mantua.
He sends Friar John as a messenger.
Friar Lawrence organises for Friar John to bring Romeo news of Friar Lawrence and Juliets plan (Act Five, Scene 1)
Quarantine.
His messenger, Friar John, was quarantined.
================================================================================================================================================================ ANSWER: In Verona, Friar John tells Friar Lawrence that he was unable to deliver his letter to Romeo due to an outbreak of an infection. =================================================================================================================================================================
Romeo fails to receive Friar Laurence's message due to the epidemic occurring in the city. Friar Laurence gets quarantined and is unable to bring the letter to Romeo.
Friar Laurence asks Friar John to deliver a letter to Romeo explaining the plan to fake Juliet's death.
Friar Laurence serves as a messenger between Romeo and Juliet. He helps them communicate and execute their secret plans to be together.
Friar Lawrence thinks it may bring about an end to the family feud. He believed it would bring the two families together again.
Friar Lawrence sends a letter to Romeo, who is in Mantua, detailing his plan to fake the death of Juliet, and have Romeo bring her to Mantua where they can both live happily ever after. He gives the letter to Friar John, who is supposed to bring the letter to Romeo ASAP. Friar John never makes it to Romeo, because he was quarantined due to an outbreak of the Plague. Therefore, he brought the letter back to Friar Lawrence. This messed up Friar Lawrence' plan, as he had already set it in motion.
Friar