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. ... The adult flies are the most common sign of activity.

Why are there so many flies in my house all of a sudden?

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.

How do I get rid of flies in my house?