Flies are harmful to human beings because they carry a variety of pathogenic microorganisms. Flies have hair on their bodies, and the foot pads can secrete mucus. They like to crawl in human or livestock feces, urine, vomit and corpses for food, and it is very easy to attach a large number of pathogens, such as Vibrio cholerae, Salmonella typhi, hepatitis B, polio bacteria, ascaris eggs and so on. Often stay on the human body, food, tableware, and have the habit of rubbing feet and brushing your body when you stop. Bacteria attached to it will soon pollute food and tableware. When flies eat, they spit out the crop liquid first, dissolve the food before inhaling, and they can eat, vomit and pull. In this way, the pathogen that was originally eaten in the digestive juice is spit out together, polluting the food it has eaten, and people will get sick if they eat these foods again and use contaminated tableware. The prevalence of cholera, dysentery and bacterial food poisoning is directly related to the spread of flies.