Considering that at any given moment 91% of the households in the world have at least one fly in it, it is impossible to say whether a fly is or is not a danger. Every insect is susceptible to being exposed to some kind of pathogen or parasite, and it is naive of us to believe that just because we've purchased our bugs from local pet shops, that those bugs are "clean" or free from any kind of danger that those common house flies might carry. Yes, flies eat crap, but so do crickets, and I don't know about everyone else, but my cricket cage smells ungodly after a couple days. I'm fairly sure that those bugs are just as funky as any flies would be. We all grow up knowing that flies eat dog poop, so we have a natural aversion to flies. Crickets on the other hand make a lovely sound on a summer night and for maybe that reason we think of them as nothing else but peaceful. Reptile owners know better though,don't we? I just have a very hard time taking things for granted, and for that reason, I think that a fly is as good as a cricket as far as protein goes. Granted, I'm only talking about an occasional fly here and there, and not changing a chameleons main food to flies. I'm saying that I just don't think we need to worry too much about our chams becoming sick from a fly or two - or five if that's what they eat while outside.