Most flies like the common Housefly are harmless nuisances at the worst. But there are a few troublemakers out there like the biting Horsefly but that usually just happens outdoors.
Then there is the Cluster Fly. These guys swarm into unsuspected homes with really no rhyme or reason we can identify. Maybe they like a plant in your yard. Maybe they like the color of your house. We really don’t know what attracts them but they will come in droves, sometimes in the tens of thousands, to hide out in a nice comfy home during the winter months. If this happens to you, call in the pros. But most of the time, the best way to avoid fly issues is to make sure they can’t enter your home. So don’t leave doors and unscreened windows open, repair broken screens in attic and crawl space vents, and do your best to seal cracks and openings leading to the outside.