We have a lot of systems that depend on one another; perhaps a severe enough pandemic would cause a sort of cascade of collapse. I'd think it would have to be really bad, though, certainly worse than killing 1/3 of the population.
I am sure there would be some collapse, the question is how long will it take to rebuild. I would imagine that the survivors would just abandon large swathes of land and concentrate themselves. Having low population density overall is not a problem -- look at e.g. Australia or Canada.
But we are now really in movie-plots land. Are you prepared for the zombie apocalypse?
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