What is the scientific evidence of flies' being dirty?

Flies feed of some dirtiest things in world: feces, garbage and carcasses of dead animals. The other thing about flies is that they are not able to chew your food! To digestive, they spill enzymes on food to digest the food. Although the produced enzymes by flies are creepy enough, but the problem is Bacteria and viruses that are clinging to flies bodies and are cause to disease in people. So even a second time is enough for flies to import all microbes on their legs and hair in your food. Since the flies can carry dangerous infectious diseases such as cholera, diarrhea and typhoid fever, it is better to do not eat a piece of food that flies sits on.