Why are house flies considered transmitters of food and waterborne illnesses?

House flies are considered transmitters of food and waterborne illnesses primarily due to their ability to engage in pre-oral digestion. This process involves the fly regurgitating digestive enzymes onto their food before consumption, which not only aids in breaking down the food but also facilitates the transfer of pathogens present in their saliva. As house flies land on various surfaces, including garbage and feces, they pick up harmful microorganisms, which can then be transported to human food and water supplies. This makes them effective vectors for a variety of pathogens that can cause gastrointestinal diseases, making the understanding of their biology crucial for vector control and public health efforts aimed at reducing the incidence of these illnesses.