This is because in the long-term evolution process, flies and bacteria have formed a mutually adaptive relationship. Scientists have found that most of the bacteria on flies hide in their digestive tract. These bacteria that are very harmful to human body can only exist in the digestive tract of flies for five or six days, and then they will die. The rest will be excreted with feces, causing no harm to flies. In addition, flies also contain an antibacterial active protein, This active protein can eliminate and kill all kinds of germs. Therefore, although flies stay in dirty places all year round and carry many germs, it is not in itself. The reproduction rate of flies is very high and it is a very harmful insect.