Flies may not make the most loyal pets, but there's no denying that they have landed with all six (eight?) feet in the English language. Their name comes from the Old English 'fleogan' (to fly), the same source as our verb 'to fly.' And fly they do -- the problem seems to be that they also land where they're not wanted. One of the earliest fly-based figures of speech, dating back at least to the Old Testament Book of Ecclesiastes, is 'fly in the ointment,' which means a small, disagreeable detail that ruins the enjoyment of something nice, like a jelly doughnut. In the 19th century, to say that 'there are no flies on him' of somebody meant that the person was alert and active, probably by allusion to cattle that move around enough to deny flies a landing place.
But when flies do land, they often do so in massive numbers, giving us the simile 'like flies,' meaning 'in huge numbers.' Shakespeare, in Henry VI, Part 2 (1595), wrote 'The common people swarm like summer flies,' and it would not be inappropriate to say that Americans are, as of this writing, watching Regis Philbin 'like flies.' Thus, although flies are indeed fragile creatures and seem to die easily, 'dying like flies' or 'dropping like flies' simply refers to death in huge numbers ('like flies'), whether of people or animals, not to any imagined similarity between human and fly mortality.
http://www.word-detective.com/071000.html
