Actually in 1918 the fact that influenza was a viral disease was unknown, the role of viruses in human disease was barely in its infancy. A great deal of effort was put in to finding the "bacteria" that caused influenza. Other than supportive care for the sick, there was little specific treatment. A large proportion of the deaths came from superinfection with bacterial pneumonia on top of the influenza pneumonia, and there were no antibiotics in 1918. Oxygen supplementation to aid support for pneumonia was not a thing either. Bottom line was supportive/nursing care and hoping the patient would get well on their own - ZERO specific therapy. Quarantine would help, as this was strictly person to person transmission with airborne mode, and this was effective where it was rigorously enforced. Note that some medieval towns did enforce strict quarantines and avoided the plague even with the rats not "quarantined".
Make the virus as infective as measles (which lingers in the air up to 12 hours in a room and will almost certainly get you) and it is possible to see a death toll approaching 30%+. Isolated and some rural areas may get lucky, OTL American Samoa was locked down and had no flu and some villages in Alaska were quarantined and had no flu get it – just examples. OTL the Black Death was a major cause of the death of the feudal system as the labor shortage and empty land meant serfs could decamp and find a better situation and the new landlords would turn a blind eye. I expect that in the southern USA those African-Americans who survived would find their situation improved as landlords who were abusive or areas that were all Jim Crow would find folks picking up and moving for greener pastures. In any case, there would be massive social changes.
Pretty much the only areas that did not have a flu outbreak were those that were isolated either by quarantine or geography. Since it was person to person any spot that did not have contact with an infected person between summer 1918 and spring 1919 (there were 3 waves) escaped the epidemic.
That leaves several interesting possibilities – certainly to my mind.
If there had been rapid population loss in Europe and East Asia, this would have destroyed the cheap labour necessary for labour-intensive manufacturing and likely sped up their current shift towards dominance by skill-intensive industries. Because dominance of skill-intensive industries requires a level of education that crowds out a woman’s prime reproductive years, this would likely lead to shifts to lowest-low fertility in Europe and East Asia at a much earlier date (say, 1930 versus 1995 or so). Such a trend would naturally have shifted global geopolitical power earlier and more decisively towards North America and the resource-rich desert states around the Indian Ocean. Observing
current environmental politics, I cannot imagine this as a favourable outcome, because the fossil-fuel-rich desert states would have been equally resistant to genuine action as they are today and much more globally powerful. In fact – since they might well have (at least relatively) escaped the Spanish flu due to their aridity and nutrient-poverty – Australia and the Gulf States might even have attracted sufficient energy-intensive manufacturing to be global superpowers, which their relative labour scarcity currently prevents even with extremely low taxes and lax environmental regulation. Such a scenario would have meant a climate catastrophe at an earlier date even with a smaller global population.
Another possibility is that Europe and East Asia would specialise in
capital-intensive industries. This is perhaps quite probable in the economy of the 1920s than it is now because:
- pollution problems were unrecognised and heavy industry unregulated
- with their populations decimated capital would become much more abundant than labour or natural resources
I am not sure what the effects of specialisation in capital-intensive industries would be, but it often seems to be a “worst of both worlds” scenario vis-à-vis specialisation in resource extraction or in skill-intensive industries.
A less likely but more favourable – at least for the planet’s ecology – alternative would see the large landlords retain a sufficient degree of political dominance in Europe and East Asia to prevent minimum wages and technology crowding out agriculture and slowing or reversing their growing comparative disadvantage in this sector – the sector which uses their solitary natural resource of geologically uniquely fertile – and geologically uniquely
renewable – soils.