U.S. no paid sick-leave a problem
AC wrote: "The biggest problem in the US is that people with no or limited healthcare don't go to the doctor and you miss the early stages on the spread which makes containing it and treating it harder."
The other thing is, many of the millions of low-income or minimum-wage workers keep on GOING TO WORK every day for as long as they possibly can in the early stages of things, because they have *NO* PAID SICK-LEAVE and they can't afford to miss any pay by taking time off from work just because they felt a little woosy or what they thought was just a cold (to *start* with, before it gets worse).
But from what I understand (generally speaking) they can still be CONTAGIOUS to everyone around them since those viruses are airborne and harder to avoid (as in, breathing).
The worker pops some cheap generic "cold medicine" (kept on hand for such occasions) to help mask the initial symptoms, you go to work and try not to let on that you feel like crap even though you're about ready to fall over, and do your best to make sure the boss doesn't notice, because you don't want to get sent home and lose a day's pay. You might be able to pull that off for some time. So you spread contagious *airborne* viruses to your CUSTOMERS and the rest of the general public - thinking of restaurant jobs, hotel/motel workers, some cashiers and the many other low-paid or minimum-wage "service" jobs that typically don't have any benefits such as paid sick-leave. It's common.
It could become a problem if a nasty enough airborne-contagious virus was involved. You'd just have to breathe a few whiffs of the same air as one of those early-stage sick workers, then you take the virus/disease home with you and spread it to your own family, friends, neighbors, your own coworkers, etc., and it goes on from there.
In the case of some new quickly-spreading highly-contagious airborne virus, seems like it could be a problem. The government tells sick people to stay home, but they go to work anyway (as far as possible) because they need the money.
I also wonder about the more urban tightly-packed populations we have now, which would speed up transmission, compared to (at least in the U.S.) the still comparatively less-urbanized populations at the time of the 1917/18 flu. You'd think we modern humans, as descendents of survivors of the 1918 flu (I lost some ancestors to that), would have some built-in genetic immunity, but maybe it doesn't quite work so neat and tidy like that, who knows.