Houseflys come from maggots. maggots come from eggs laid by flies on rotting meat, and when meat is absent, rotting fruit. What was dead, emitting a foul odor, that generated a persistent fly problem? If it was dead rats, there needed to be a constant supply. If a dead raccoon or squirrel- same issue. As slidey as this is, makes me wonder wonder what the source of the smell was. Q asked about renovation in a much earlier post, and a wall was removed.
Oh I forgot to add that housefly like to land on and congregate on feces. XD