In the movie "The Green Mile", John Coffey heals the prison guard Paul using his special power. He then opens his mouth and hundreds of "flies" appear buzzing away.
What are those flies supposed to represent?
Coffey also exhales insects after he cures the mouse with the broken back, so I don't think the flies can represent disease extracted from the person being cured.
My guess is the flies are something like the exhaust gas from a motor. Whatever mechanism Coffey uses to heal generates (psychic?) waste that he has to get rid of. However I can't find anywhere in ther book that goes into any more detail. In fact I can only find the two mentions of the flies.