The common housefly, Musca domestica, lives in close association with people all over the world. They feed on human foodstuffs and wastes where they can pick up and transport various disease agents. In large numbers flies can be an important nuisance by disturbing people during work and at leisure. Flies soil the inside, and outside of houses with their faeces. They can also have a negative psychological impact because their presence is considered a sign of unhygienic conditions.