My population (in excess of 500,000) undergoes a huge dieback event, almost extinction. Narrative necessity (possibly) dictates this.
I've run through the possibilities and disease is the only feasible cause in this case. Other suggestions that fit with the requirements, are fine though.
Is there a disease or potential treatment for a disease which could result in (large-scale death and/or) memory suppression or memory alteration?
The memory loss may only need to last a matter of months, societal collapse and grief may take care of the rest.
Disease treatment resulting in additional memory loss could be acceptable here.
Setting: In term of medicine, equivalent to England in the dark ages, ie people were uneducated and prepared to try anything.
My understanding is that some psychotherapeutic treatments have been accused of creating false memories. There is no modern psychotherapy in this world, answers should take into account social behaviour of a non-scientific society which has no single organised advanced medical provision.
There is trade with other areas, many plants/medicines may be touted by "snake oil" salesmen, not to mention tics, parasites, reptiles etc maybe hitching a ride.
Any plant/animal toxin may be taken into account and a mixture of those would be fine if it has the desired effects.
No asteroids, environmental collapse, nearby nova events, sun eruptions or geological events will be considered.
The Key: Whilst certain memories may be available to the population with effort of recall, these memories will no longer seem important or influence their behaviour.