Outside your home they will leave their eggs in garbage cans, dumpsters, compost piles, human and pet excrement, open dumps, sewers, culvert systems, food processing plants, and rotting organic material. If you live within a hundred feet of any of these, you'll have flies.

How do flies come out of nowhere?

The most common reason for flies swarming all over your house is an infestation inside or nearby your home. If you suddenly see a swarm of flies that means dozens of eggs have already hatched and developed into flies. The source is likely inside your house, garage, attic or garden.

What causes a lot of flies in the house?

How did I get house flies? House flies are one of the most common insects on the planet. They breed and feed in filth, including garbage, feces, and rotting or spoiled food. Poor sanitation and ripped screens and unsealed cracks in windows and doors can lead to house fly infestations.