Just how close ???

Among the flies are some of our worst enemies. Some species destroy crops; they live as parasites under the skin of animals, causing myiasis (infestation of living tissue with fly maggots); and they carry diseases

Most species are harmless to humans and play an important role in the balance of nature. They carry pollen to plants; they are an important link in the food web; and they hasten the decomposition of animal carcasses, manure, and vegetable matter. In addition, they consume a great many other insects. About 5,000 species of tachina flies prey on other insects, and are sometimes used as biological control agents in controlling injurious species. Flies are an important food source for many animals such as frogs and toads, lizards, and birds.