[1] If people are dropping like flies, they are dying or falling down in large numbers: (像蒼蠅落下)
死了很多人；跌了很多人...
[a] The heat was overwhelming and people were dropping like flies.
[2] to stop doing an activity in large numbers:
[a] There used to be over 20 of us in our aerobics class but they're dropping like flies.(像蒼蠅落下)很多人停止做活動...