1
general::
Phrase(s): drop like flies
Fig. to faint, sicken, collapse, or die, in great numbers like houseflies dying in a large group. • It was a terrible year for the flu. People were dropping like flies.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms