Who's to blame?

During the Middle Ages not much was known about germs or infection. People knew that once one person got sick there was a good chance his neighbors and friends would be similarly afflicted. The only remedy they could come up with was to lock sick people up in their houses until they either died or got better.
In lines 5 to 12 of this scene Friar John (the man who was supposed to go to Mantua to alert Romeo to the ruse that Friar Laurence had devised) tells of being locked in a house with sick people.
The result was that Romeo never got the message. I'd like you to assign blame for this (fatal) lapse. Who screwed up? And please explain exactly how you think this person screwed up.
Answers of one sentence or overly short answers will not receive full credit.