I've gone through this the past couple of years. When I'm vigilant for a few days, the problem ends. What I mean by vigilant is, every room, checking every window, and killing every single fly. I admit it, I don't understand why it keeps happening.

Is there an exhaust fan in the bathroom?

Sign of the devil. Been to church/temple lately?

JK! but . . .

This happened to us, several times now. Different homes.....as follows....

#1) I opened the latch on the crank-out windows to crank the windows open. They came in the crank handle holes. I swung the crank back....filling the tiny hole...no more flies. I had 100 in the house in one evening, before I found THIS out. After that ZERO flies.

#2) 4-6 flies & 12-14 miller moths every day ! The cat was eating the moths making him sick. Vet said they are too rich for cats to eat. :0( We were in the loft bedroom, looking up at the pretty beams....and saw a 4" x 6" hole above the beam, no other way to see it unless your laying down, looking up. Filled it in with insulation, then patched with drywall. Now we have ZERO bugs of any kind!

Use your imagination....it's nothing obvious....and....we NEVER saw a bug entering the house. But after the fix.....0.

The exhaust fan is the source however if the weather is about to change they will fly in when the doors are opened. Only way that they would have come from the plumbing is if something laid their eggs in it a long time ago but you would probably have noticed.

Double check to see if all the windows are completely shut and latched. I have had windows slide open and although they looked closed they were not. However I thought you slept with a window open in all kinds of weather.

Are they regular house flies? Or perhaps they might be drain flies. Different pest. I had drain fly infestation some years ago -- had never heard of or seen them before. Smaller than house flies.

The solution to getting rid of them was to spray WD-40 into the drain AND the overflow hole of the basin. Problem eventually solved! Yay!

House flys live up to a month. You still probably have some from the dead animal you found.

Up at camp we are surrounded by woods on 3 sides and a cow pasture on the 4th. We had every kind of fly imaginable. House flys, horse flys, and deer flys. We couldn't sit on the front porch without them using us as a meal. I found an easy device to keep them away from us. Take a Mason jar and put in some raw meat and water to just below the meat. Put a funnel on the Mason jar wide end up and secure. I use caulking. The flys are drawn to the raw meat and enter the funnel and can't fly back up and drown. I put them outside in an area that is somewhat protected from the rain. Since then we haven't had any flys in the house or been bothered by them outside.

Flies are the bane of my existence. I know where mine come from...the kitchen door which has to be held open to let dogs in and out. I swear it's like I live on a farm. I'm going to look for sticky flypaper that I remember my parents using when I was a kid. I don't use insecticide. They tend to end up in my sunroom where I chase them with my fly swatter usually knocking stuff over in the process.

DD had drain flies from the rarely used bathroom shower in her guest bedroom suite. i poured several teakettles of boiling water down the drain, and that took care of it.

At this hot time of the year, they swarm around our doors and seems every time we go in and out, one or more flies come in! Last year, on a particularly hot day, I counted 18 flies that I killed. So, maybe they are just flying in if you leave the door open for a while, and gather in the bathroom because they like it there. They head for the windows because they want out and see the light and think they can get out. I hate them because my dog is afraid of flies...HAHA!

Thanks, all! I think greywings has hit on the source. The exhaust fan for the bathroom vents above the window well where the animal was decomposing. We're down to only two flies this morning. *Maybe* this will stop, now that the souce is gone, but if not, I guess we need to more tightly screen the vent -- if that won't interfere with the fan function. I'll go out and look if it stays dry here long enough to do so today.

function -- Good information about how long these pests can live. I hope there's not some 'reserve supply' of them IN the fan vent! I'll try running the fan to evacuate whatever might be in there. It's a pretty powerful fan, sized for this large room.

Yeah a dead animal, usually a mouse in the crawlspace (or window fan in your case) which attracts one fly from the outside then it's eggs hatch into maggots/flies which then seek out a new food source inside the house. I've used several brands of fly paper over the years but they only catch fruit flies, someone needs to invent a flypaper which is treated with a housefly attractant and not just be sticky.

I've had 2 incidents of cluster fly outbreaks, both times in my family room. The first was in 2012. This spring was the latest, fortunately lasting only 4 days or so. Apparently this type of fly likes to overwinter inside house walls, then emerges when it is warm enough. I haven't been able to find the opening they used to get inside the wall from outside, nor one that allowed them into the interior. But this 80 year old house probably has multiple tiny gaps that I just don't see.

This is the one thing I dislike about Summer. Flies in the house. Have only seen one so far this year but they seem to love my daisies that are in full bloom now.

Ta-Da! The fly invasion appears to be over. It had to have been due to the proximity between the decomposing animal in the window well and the vent for the bathroom exhaust fan. Kudos to Graywings for suggesting the exhaust fan connection, and thanks to all who responded. I've disassembled the central vac's 'bazooka' extension and put the hose away. DH can wait a month before changing the central vac's bag -- in light of Function's information about the lifespan of flies! He did that chore recently, and we normally only need to do it quarterly.