What could I "use" to cause a population wide long-term memory loss.
So far all chemicals that I have read about cause future memory loss. That is to say, if you are exposed to it today, tomorrow you won't be able to form new memories.
But what I need is a chemical of sorts that will cause a population to forget anywhere between a few months to a year in the past, depending on how strong the exposition to the chemical was.
It needs to be propagated by air so that not everyone will suffer the same effects, preferably.
Does such a thing exist or should I just make-up one?