Houseflies are commonly found where people work or live because of the warmer environment and ready supply of food. Bluebottles (sometimes called blow flies) are also often found in human environments, and are particularly attracted by meat and decaying materials.
Where do they come from?
Eggs are laid in moist or rotting matter - household rubbish, compost or manure and once hatched the flies reach maturity in anything from two weeks in warmer weather. Common houseflies have a flight range of five miles or so, and can easily transfer from the breeding grounds to home. A sudden appearance of bluebottles in the home normally indicates that a small animal (maybe a mouse or shrew) has died - possibly under the floorboards or up a chimney. Female bluebottles are easily able to find sources of suitable food, and are often found in domestic kitchens.
Why do flies come indoors?
Houseflies and bluebottles come indoors looking for food. They are not fussy what sort of food they settle on, and are likely to be highly active once indoors.
Can they do harm?
Houseflies can transmit intestinal worms, and are also known to be carriers of particular diseases - gastroenteritis (tummy bug), for example. Bluebottles too, are known to spread diseases, and certain precautions should be taken. Because of the way flies feed and where they are likely to have come from before settling on the food, it is as well to make sure that you cover any food to avoid contamination.
You should frequently clean drains, particularly near kitchens, and cover your waste bins to avoid providing ideal breeding conditions.
How do I get rid of them?
The best ways of controlling or avoiding infestations are good hygiene and taking the simple precautions mentioned above. Insecticide control using fly sprays ("knock down" sprays) are a good, almost instant, way of dealing with the problem. Flypapers, while unsightly, are another solution. Electric ultra-violet fly killers are another means of controlling flies, but their efficiency can be affected by where they are placed.