Although there is a family of flies known as house-flies it is actually the family of blow flies that can be the biggest nuisance to human kind. Blow flies include the common bluebottle and greenbottle with which most people are familiar but these is also the cluster fly (Pollenia rudis) that falls in to this category.
Unlike some of its cousins that lay eggs in carrion (and that includes unprotected meat in our homes) the cluster fly actually predates earthworms. However, in autumn they 'cluster' together to find shelter for the winter and that can be in our houses. The arrival of a cluster of cluster flies is not to be welcomed! they can be a considerable nuisance rather than a danger to health. They tend to inhabit woodland in summer where they breed so it tends to be houses near woodland most at risk of an autumn visitation.
Identified not only the the sheer numbers but also by the chequered abdomen and golden, furry thorax.