One main reason for dead flies in a home is that flies enter the home through cracks and crevices, then are unable to gain access to the outside again, thus ending their life cycle inside the home. Flies generally seek indoor environments for warmth or for obtaining food.
Though some species of flies are only a nuisance in the home, others can cause serious problems. Some flies feed on garbage and animal refuse, and then carry bacteria that can make humans ill. Other flies attract insects when they die. These insects feed on their carcasses and create their own problems.
Controlling access to the home in the fall is the best method of preventing dead flies inside. A home's southern or western exposure is most vulnerable to flies. Caulking cracks around doors and windows can prevent entrance and is most effective when used with an insecticide sprayed in the fall. Killing flies that congregate on the side of the home during the spring is not necessary, since this is a sign that they are leaving the home.
Once indoors, some flies remain in the walls until the spring. Others come out of the walls and fly around when temperatures rise. These flies can be killed with a flyswatter and vacuumed away.