Housefly fungus

From Wikipedia:
Outbreaks of infection with Entomophthora muscae tend to occur in the spring and autumn. The fungus is found in most temperate regions and sporulation usually takes place in cool, humid conditions in areas where flies congregate and rest. In houses, the corpses of flies are frequently seen attached to windows or window frames, a place where other flies may easily become infected. In the open, they may be seen attached to the underneath of leaves, on fences and walls, in agricultural buildings and poultry house.