drop like flies - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Rapidly collapse, die, or drop out, usually referring to a group rather than an individual. For example, The words were so difficult that the spelling bee contestants were dropping like flies. The simile like flies has meant “in large numbers,” as it does in this expression, since about 1600.