The eggs are laid in damp areas on any type of rotting organic matter: dead animals, animal feces, kitchen waste, rotten fruits and vegetables, grass clippings, compost, and so on. When they move on after egg laying, adults carry away microorganisms and particles of organic matter that may be harmful to human health. They carry them on the fine hairs on their bodies, the spines on their legs, the pads on their feet, and their sponging mouthparts.
Given the nature of their mouthparts, houseflies cannot consume solid food. They must first dissolve it by regurgitating a liquid onto the food. Since their stomach contents come mainly from rotting organic matter, the regurgitation behavior is yet another potential source of contamination. In fact a fly’s stomach contents will either be regurgitated at the next meal, or excreted in the form of feces, often on food that will be subsequently eaten by humans.