None of us are immune to flies as long as we are constantly exposed to them on a daily bases, either in our homes, in restaurants, food market, hospitals, city parks, rural environment, working environment as we travelled, visits friends and family in different homes and neighborhood.

Flies are usually found in areas that are dirty and filthy, they live almost entirely on filthy environment. They breed on all kinds of decaying substance, like human waste, manure and waste from animals like cow, pigs, chicken, pigs etc.

They are also found around waste and garbage deposits. Each female fly is capable of laying over 1500 eggs. These young flies are then ready to start the cycle all over again thereby increasing and multiplying there population considerable.

source: http://bnetlens.com.ng/dangers-of-housefly/