Dropping like flies
Falling down ill or dead in large numbers.
The origin of this phrase isn't known. It is clearly a simple allusion to the transitory and fragile nature of an insect's life. It is known from around the turn of the 20th century. The earliest printed version I have found is in The Atlanta Constitution newspaper, May 1902:
"I saw men and women rushing back and forth within the flames. They would run along, then came the choking smoke and they would drop like dead flies."
In the early 19th century the Brothers Grimm's published 'The Brave Little Tailor', which is a cautionary fable of a child who easily and thoughtlessly kills numerous flies. It seems that they chose flies as being synonymous with something even a child could kill with little effort. The phrase doesn't appear in that text.