The most common observed stage of a fly is the winged adult. The adult fly mates, lays eggs in a breeding medium that will provide sufficient food for the immature stage--a pale, legless maggot. The breeding site is nearly always moist and surrounds the soft-bodied maggots. When maggots are full grown, they stop feeding and usually wander from the breeding site in search of a place to pupate. After pupation, they emerge as an adult fly. In warm weather, flies complete their development (egg--larva--pupa--adult) in an incredibly short period, 7-14 days, and produce numerous generations during a typical season.
Because animal excrement and garbage are excellent breeding media, certain flies, especially house flies, can transmit disease pathogens. For example, it has been shown that each house fly can easily carry over one million bacteria on its body. Some of the disease-causing agents shown to be transmitted by house flies to humans are: shield spp. (dysentery and diarrhea = shigellosis), salmonella spp. (typhoid fever, Escherichia coli, (traveler's diarrhea), and Vibrio comma (cholera).
The best way to prevent fly problems in a home is to exclude them by screening.