One of the most familiar and widely distributed of all insects is the house fly. Besides being a nuisance, it is a prime carrier of disease. Its entire body swarms with millions of bacteria. Typhus, dysentery, tuberculosis, and poliomyelitis are only a few of the illnesses for which it is a carrier.
The house fly is found everywhere in the world. There are about 30 000 species of flies in the world, but only a few hundred are threats to human health.
Flies multiply at an enormous rate; it takes roughly two weeks from the time a female hatches until she is able to lay eggs of her own. Favourite breeding sites are horse and cow dung, exposed human feces, 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.