Flies pose high health risks to humans. House flies transmit a wide range of diseases including salmonella, dysentery, tuberculosis, cholera and parasitic worms. During emergency situations, when hygiene conditions are less than optimal, flies often are the main reason for cholera and dysentery epidemics. But even under normal conditions, children and elderly are at a greater risk of conducting more common diseases such as salmonella food poisoning from fly contaminated food.
Flies spread diseases because of their breeding and feeding habits. Bacteria from where the fly usually feed on would get stuck on their mouthparts and footpads eventually spreading onto places they land on. Imagine if it’s exposed food that you are about to eat.