Given a housefly has landed on your food, and that you then consume it, what is the likelihood of contracting some malady directly attributable to the fly?
I'm aware that flies can carry diseases such as E. coli and have even been used as a form of biological warfare, but I'm unaware of any studies showing how likely it is that your average city-dwelling housefly landing on your food will deposit active microbes or parasites, and whether a single fly can carry enough toxic bacteria to affect a healthy adult / child, as all the studies I've seen seem to study disease communication by swarms.
(Note that I'm mostly interested in the Housefly, but if there's studies and conclusions available for other species that would be welcome too)