Forgotten Wisdom From a Mashgiach Kashrus
Remember those fishmongers selling live fish? Now everyone has refrigeration, and the fish is packaged or at least hidden behind glass displays. But it is important to know why they used to have those noisy electric fans with frills attached. There exist (black and green) flies which haunt fish chicken and meat, especially during summer time. The flies inject their sperm into the flesh, which in a matter of hours gets worms.
Either that or my friend is pulling a prank on me…
Bottom line: Never leave raw fish or meat outside of the fridge unattended.
Update: I found out these “meat flies” (not this, I think) are already mentioned in Darkei Moshe Y.D. 86:187.