About the flies - fly screens fitted to all the windows works a treat. They're pretty much standard issue in Australia. Of course, it doesn't help when you're out and about but only the occasional fly manages to sneak inside when a door opens.
In the summer we don't open windows at all (humidity + heat = uncomfortable house), and the front door has a screen door, but it only takes one pregnant fly coming in to give you a colony.
First, I've never seen a home without screens on the windows, even both sets of grandparents, whose homes were built in the very early 1900s. (US)
Also, Diane, I'm wondering where in a home a fly could have laid her eggs. I really want to know if I am missing something. Thanks.
Probably in a sink drain. Pour boiling water down every drain in your house. I'd repeat it everyday for a week or so, just to be one the safe side.
From a website: How to make a fly trap...
Making a fly trap
Cut the tops off several plastic water bottles and invert the tops into the lower portions, forming a funnel. Put about 2-inches of apple cider vinegar in each bottle with 1/4 teaspoon sugar. Almost all flies, no matter their normal food preference, will enter the trap. Death by drowning or by acid--sugar though is the lure. Flies aren't so different than humans after all.