Flesh flies, otherwise known as sarcophagus flies feed on dead or decaying tissue. They're in incredible asset to forensic entomologists because certain species of Sarcophagidae will lay eggs OR live larvae on the body at a certain stage of decay. In other words, some flies like it warm. Others like it cold, and yet others like it nice and mushy. But based on the life cycle of the fly maggot/pupae, forensic entomologists can determine when the individual died.
See a lot of flies like these buzzing around? Chances are that you're at a farm (they eat manure too), something died (like that mouse in the wall of your basement), or your neighbor is hiding something.