" People don't live merely to survive: we're hardwired to propagate our genes. If you cannot think abstractly and articulate your ideas well, you will have difficulty attracting a mate. People who have disabled their ability to examine themselves will be quickly eliminated from the gene pool. Hence, it seems unlikely that such an illness will occur because it goes against how natural selection has shaped us. "

I don't disagree with the gist of the above. However it is tricky to assign clear intentions to a non-human agent, assuming one views biological undercurrents as an analogue to an agent in the first place. Which brings us to:

" This reasoning seems to rely on the assumption that the mind was designed by some kind of agent. Who do you think is deciding whether it "makes sense" to allow an expansion of the ability to think? Our best theory is that cognitive expansion resulted as a series of mutations that improved the ability of our ancestors to survive. One does not need to appeal to the fact that "Day Zero illness" does not "make sense" to argue for its implausibility. It is implausible simply by the fact that it is a priori highly unlikely for any novel previously unobserved phenomenon to exist in the absence of a very strong theory that predicts it. "

If I assume such an illness can exist, it doesn't mean I can pontificate on the way in which it would be triggered. Certainly some mental illnesses seem to be more common in modern times - despite the ability to account for them and measure number of patients more efficiently. Some slightly related illnesses that do exist are those which have aphasia as a core part. Usually in pre-modern times one finds more elaborate personal accounts by poets and other authors, of such sensations or states; eg in the case of an aphasia-like state, there are two good examples, one from Baudelaire (the french poet) and his sense that he was "touched by the wing of idiocy" etc, and the very dramatic story of the deterioration of Guy De Maupassant (important story-writer), who in the end "reverted to an animal state".

However, as I noted, the hypothetical illness I wrote about is not just an individual case with elements of aphasia. Primarily my background for asking the question has been that any human is not primarily (in my view) an outward/social oriented being, but in the vast majority of cases humans are indeed social agents (due to a variety of reasons; usually having to do with clear rewards). However, below all that there is the person in their world of consciousness, as part of the greater world of the mind. It may be, therefore, that a risk can be picked up (more on by what it will be picked up later) as serious enough if it somehow attacks the inner world, that even a massive exodus from formations about anything closer to a surface (like interests in the external world) may occur. In such case, assuming it is possible, it would be easier to cause not a full erasure of memories or skills, but a negation of the ability to stabilize them, as briefly presented in the definition of the new illness in the OP.

As for your point about all this having to allow for the mind being created by an agent - no, that isn't so. I certainly have no reason to think the mind was created as a set work, nor (of course) that it existed a priori or may be sensed as existing a priori even figuratively. The way in which it developed (mutations etc) doesn't by itself have to cancel the possibility of a non-yet seen illness appearing. After all, as you agreed, not much of the final (such a thing cannot even exist) form of a mind can manifest, given this system of connections cannot exhaust all its possible rearrangements during the person's lifetime (likely not even if the person could live for 1000 years). I do approach this from a more literary (which, sadly, at times means even less literal...) point of view, given literature and philosophy is where my interests and studies lie.

I should also give at least one parallel (it won't be perfect, and it may lead to problems as well...) with a procedure which allows for a new development on a larger scale, while it wasn't picked up individually up to then. Given that if something like the DZI would exist, it wouldn't have been picked up before, it can be said that what was doing the picking-up or noticing certainly would not act on the same level as an individual (eg some individual sufferer or some aphasia-like condition). This would perhaps be possible, if the complexity of both the trigger and the formations which pick up the trigger were again far larger. In effect, in my hypothetical, the general idea was that some core pattern or patterns - not created by any agent; not conscious and not accounted for - does exist, which would signal due to special relation to the unconscious mind some particular and grave danger. Such patterns do not even have to be intelligible to an individual in the first place. In that, perhaps, it deteriorates somewhat to the realm of fiction; yet most complicated patterns aren't making a full impression on someone who views them. In fact we can be said to be surrounded by patterns which are not picked up, due to our position or lack of related interest to notice. Maybe - that is the hypothesis - a slight difference will lead to the unintended formation of a curious pattern which happens to be related not to the thinker but to some scheme in the mental world. After all - here comes the parallel - it isn't rare to see the opposite happen, for humans project math formations into external objects (eg the fibonacci and other φ related patterns, on shells etc). If we can project math onto the external world (which isn't anthropic or mathematical; math is not cosmic, in my view), why shouldn't some formation there present us with other elements and balances of our own mental world?

That such would be catastrophic, or cataclysmic, is just an assumption.