One of the most familiar and widely distributed of all insects, the housefly, besides being a nuisance, is a prime carrier of disease. Its entire body swarms with millions of bacteria which are often transmitted to the food we eat. Typhus, dysentery, tuberculosis, and poliomyelitis are only a few of the illnesses for which it is a vector.
Flies multiply at an enormous rate. It takes roughly two weeks from the time a female is hatched until she is able to lay eggs of her own. Favourite breeding sites are dung heaps,exposed human faeces, all sorts of droppings, rotting garbage, and carrion. In more developed countries, modern sewage systems, refuse removal, and general cleanliness have had a marked effect in controlling the insect’s numbers.
Source: Canadian Museum of Nature