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I am currently writing a book about a pandemic. If you want more informations, see my previous posts here and here.

Imagine a bacterium with a very high infection rate, so that within a year, most people (>50%) are infected.
If the disease had a mortality rate of n, around n*N people would die within the year (N = total Population).
Perhaps a few less, because of herd immunity etc.

However, when enough people die, this is no longer true. When there is no medical staff to treat infected people or more importantly not enough people working in food production, or important and irreplaceable people like high level politicians die, more people than predicted by the above equation would die.

Our food production and society in general relies massively on efficiencies of scale. Meaning our society could exist in the same way it does now, if only half as many people lived, but the proportions of professions stayed the same.

So my question: How many people would have to die (or be sick enough to be out of work), for society as a whole to fall apart? Are there any historical examples for catastrophes on this scale? (natural disasters etc count too)

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Qoray
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    I would offer you an answer in re the Mayan civilization's fall, but do not have enough meat on that bone to do this question justice to give you a model, and some of that is still speculative. What civilizations have you looked into so far? – KorvinStarmast Mar 02 '20 at 21:50
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    This is not so much about the minimum viable population as about the resilience of society to the losses. 1% deaths among active members can send the society into tailspin. – Alexander Mar 02 '20 at 21:50
  • That is no problem, I appreciate any suggestions. The model is not all that important, I just want to know, how deadly I have to make my disease, without being too unrealistic, to seriously affect day to day life. – Qoray Mar 02 '20 at 21:58
  • (The antagonist is a Ted Kaczynski type of character, who wants to kill people, so that we go back to preindustrial times) – Qoray Mar 02 '20 at 21:58
  • Thank you very much, Morris :) That is indeed very helpful! – Qoray Mar 02 '20 at 21:59
  • I have looked at most major outbreaks in recent history (SARS, swine flu, ebola, spanish flu and the plagues), but of course, none of them were particularily dangerous (apart from spanish flu and the plague), and the others did not seem to affect society on a long term scale. In my opinion, our society during the plagues was not complex enough to be comparable to a modern outbreak – Qoray Mar 02 '20 at 22:01

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Well over a third of the population

When we studied Thucydidies a few decades ago (staff college) the plague in Athens during that time period of the Pelloponesian War (400 BC-ish), during a siege, was a source of some mystery in re "what disease was this?" (was it a novel virus, bacteria based infection?) - a mystery that was being slowly solved thanks to some deep research. (I had some articles on that which are either in the attic or got lost in a move)

What struck me at the time that we studied this stage of the 30 year long war was that almost a third of the population of Athens was thought to have expired thanks to that plague - and Athens still survived. Granted, they didn't win the war.

For your purposes, and considering the social dislocation in Europe during various waves of the Black Death / Bubonic Plague, a threshold somewhere between 1/3 and 1/2 of the popluation succumbing looks like a decent tipping point ... but you'd need to include a few seasonal and climatic factors, as well as tech factors.

Diseases don't happen in a vacuum. Some cultures and societies are more reslient to stresses than others.

Don't assume a uniform distribution of deaths: assume spikes

I'd suggest not assuming a uniform distrubution of death across the population: that assumption would actually make it easier for a social group to whether the storm. I'd look particularly at the problems of how urban population concentrations are far more susceptible to large death spikes and spreading of the disease.

This part of my answer is informed by the book Plagues and Peoples enter image description here
(which I read about nine years ago). It describes the danger of urban concentrations in spreading disease, but also how the benefits of urban concentrations can achieve the critical mass of brain power (in a cultural development sense), and economies of scale, to figure out how to cure / innoculate against a disease once a certain level of tech is achieved.

Part of your answer to your question form a world building perspective is: what level of tech is the society/culture that I am working with?

As an aside: if you do want to get your hands on that book, make sure that you get the edition that includes the treatment of the HIV/AIDS spread. The original edition was published before that was as well understood as the later edition, which has a section devoted to that particular disease.

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KorvinStarmast
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    It's going to be a very luck-based thing. If the exact wrong set of skills is removed by the plague, the community could well no longer be viable. Smaller communities will be more susceptible to losing the required skills. For example, if the black-smith sand the millers both go, your community is done for without replacements. And replacements will be hard to get during a plague. Where those two groups might be far less than 1/3 the population. Might be as few as six people, the master, journeyman, and apprentice in each skill. – puppetsock Mar 02 '20 at 22:00
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    @puppetsock well said. I may be able to fold that into the answer in a bit, but RL has just summoned me. – KorvinStarmast Mar 02 '20 at 22:04
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    Thank you very much. That was very helpful :) Do you think, the minimum viable population would be the same today as back then? (My idea was, that today, less people than ever work in food production, and society has become more and more comlpex, so there are more "attack vectors" to damage it) – Qoray Mar 02 '20 at 22:04
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    @JonathanBöcker Yeah, the attack vectors hitting particular sectors of the society will have non-linear impacts. (Medical care givers being one such sub group ...) – KorvinStarmast Mar 02 '20 at 22:06
  • @puppetsock Because we live in a global civilization, luck should basically be averaged out. I don't exspect all supermarket cashiers to get hit, and noone else :) (Of course, there are still weak points, who are not replacable, like politicians etc). – Qoray Mar 02 '20 at 22:07
  • @JonathanBöcker Heh, I'd say politicians are infinitely replaceable, but that may vary with location and governmental/social style. – KorvinStarmast Mar 02 '20 at 22:09
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    @JonathanBöcker You asked for historical examples. In some communities, as few as six people dying at the same time could wipe out the community. In others, as Korvin has shown, thousands can die and the community goes on. – puppetsock Mar 02 '20 at 22:09
  • I am not a big fan of them either, but at least in a democracy, you would need reelections etc, which cost time. You can not just hire other people, like you could as a normal company. – Qoray Mar 02 '20 at 22:10
  • oops, sorry, I didn't figure out the dupe. sheepish look – KorvinStarmast Mar 02 '20 at 22:13