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Whilst not strictly relevant to this question, the questions and answers may interest the curious: this is the 4th in a series of question here - 1, 2, 3.

• 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?

Caveats:

• 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.

Invasive Toxic Mold

Toxic mold has been known to cause memory loss, death, and symptoms like depression (which could play into why certain memories don't seem important anymore). If some vein of toxic mold developed into something much more potent, so it is able to spread quickly around and generate a lot of spores, it could conceivably kill much of your population and impair the cognitive processes/memories of the rest of the population.

To make the effect strong and temporary, it would act like an invasive species, blooming into the environment until it wiped out whatever part of the ecosystem it depended upon, and then die out. Or, maybe since it's a mold it just depends on abnormally high levels of moisture in the environment -- maybe the place experiences a flood or series of heavy storms which kill many people and dampen the environment for an extended period of time, and this mold blooms in the aftermath killing or impairing everyone else. Then, when the place dries up and the mold dies out, the survivors recover.

Dinoflagellate toxin illness.

In the late 1990s there was concern about an organism called Pfeisteria. These are dinoflagellates related to the organisms that cause red tide. Supposedly these are predatory, attaching to much larger organisms like fish, giving off the toxin to kill them, then feeding on the decaying remains. There was a bloom of these organisms and the toxin became airborne, causing illness in humans. The thing I remembered about this episode was the memory issue aspect which was the worst thing about the illness.

Patients with exposure to Pfiesteria toxin have developed an illness,
Pfiesteria-human illness syndrome, characterized by skin lesions,
headache, myalgias, conjunctival irritation, bronchospasm, abdominal
pain, secretory diarrhea, recent memory loss, and difficulties with
number sequencing. Not all patients demonstrated all features of the
syndrome. .. Until the Pfiesteria toxin(s) is isolated and
characterized, and laboratory diagnostic tests are available,
physicians must be able to recognize Pfiesteria-human illness syndrome
and intervene when symptoms, particularly memory loss and diarrhea,
cause significant impairment in daily activities. There are no
precedents for the treatment of Pfiesteria or any dinoflagellate
toxin-related human illness reported in the literature.
https://www.ncbi.nlm.nih.gov/pubmed/9524412

In your fiction there could be a bloom of these things. You could make it be an epochal extinction event type thing - some deep sediment was disturbed and the ancient dinoflagellates escaped and found a world renewed for their eating. The air and waterborne algae paralyze all animal life and then eat what dies and are spread as spores.

All the humans get infected and all have the symptoms. Most die; a very few do not.