Around 10 mm long, the House Fly is the most common pest in buildings and is found in most homes. They feed by vomiting saliva on to the food surface, treading it in and sucking up the resulting liquid! The fly is a health hazard; it passes on dangerous germs from the last place that it visited.
Which could have been anywhere from a dustbin to animal droppings!
Scrupulous hygiene and prompt disposal of all rubbish will discourage flies. Keep food covered and site dustbins away from doors and windows. You can also fit fly screens to doors and windows. There are a number of fly-killer aerosols on the market as well as impregnated plastic strips giving off insecticide vapour.