From Longman Dictionary of Contemporary Englishbe dying/dropping etc like fliesbe dying/dropping etc like fliesinformalMI used to say that a lot of people are dying or becoming ill → fly
Examples from the Corpus
be dying/dropping etc like flies• Our kids are dropping like flies.• The men were dying like flies, of fever.• Grocer profits While other retailers are dropping like flies, supermarkets are making fat profits.• They should be dropping like flies, but that hasn't been the case. 