Overall it's rather a question of water management, hand washing and such : collective and individual hygiene rules.

What is immune memory but acquired hygienic discipline at the organismic scale?

well as I understand it, if we had a collective immune system it would take the forms of smith agents cleaning your hands during the ID check.
Or an army of physiobiognomists killing infected peoples in the streets, kind of sanitary pass on steroids..

Good call with the hygiene point, I recall that tools weren't washed during surgeries not long ago and instituting hygiene as good practice actually needed to be taught early on

I think medicine did a lot of optimization on top of that, like cancer treatments, occasional treatable epidemics, or serious injuries, but the core collapse of mortality/rise of life expectancy is really from general hygiene, it's a fundamental point.
Now we finally start to understand that we should be in a kind of equilibrium with the collection of living organisms that we bath in. Hygiene is not eradication of everything that could be a menace, it's far more subtle than that.

I must say that counting a noted hygienist as a paternal grandfather and an epidemiologist as a paternal uncle - both deceased for decades - made me quite sensitive to what's abusive in the manner of many who hadn't before covid given a thought to either field, to interpret their discovery in the public scene of such specialists' concerns, as that of inventions in the pursuit of political agendas.