... I did it because flies are unhealthy insects.
That's a matter of perspective ...
Flies only try (very successfully) to survive on this planet as we do as well. Considering all the pollution caused by us (Homo sapiens), we are for sure the most "unhealthy" animal on earth.
You're right @jaki01, man is the main cause of contamination, also God created each animal for some reason.
As a scientist I claim that all creatures which are living on earth today, were created by evolution.
Of course you are free to have a different opinion (and in addition you may claim that evolution was invented by ... God). :)
Anyway, the universe, earth, nature, all the different organisms are really fascinating, I think we may agree.