In 1918 through 1920, an Influenza pandemic colloquially named the "Spanish Flu", ravaged the world. It infected about half a billion people, and killed as many as 50 million people. But my question is - how did the 1918 flu virus disappear in 1920?

One can hardly believe that after infecting half a billion people, the virus was contained in any sense of the word. Why didn't the more than one billion uninfected people get infected? Why didn't we see some small epidemics continue to circulate the globe infecting the yet-uninfected as well as newly born people for many years - potentially until today?

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    $\begingroup$ What research have you done on the epidemiology of flu virus infections, before posting here? The Spanish flu was no different in that respect from the Asian Flu of the 50s, or last year’s flu strain. (Some would say that epidemiology is off-topic here, as it is some distance from biological mechanisms, but if you ask questions here you need to demonstrate that you have researched them first. ) $\endgroup$ – David Feb 16 at 21:32
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    $\begingroup$ I tried to see if there was any similar question on this site, and found none. I tried to Google a bit and didn't find any answer. I thought that even if an answer could be found on the Web, it wasn't obvious (at least to me) how, and it would be useful to have an answer here, on this site, for posterity. If "epidemiology is off-topic here", were is it not offtopic? Please note that I wasn't asking why the Spanish Flu is no longer a pandemic. I was asking why it went completely extinct. Is there something in the flu virus which makes it a particularly bad surviver compared to other viruses? $\endgroup$ – Nadav Har'El Feb 16 at 21:45
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    $\begingroup$ Another clarification: I know there is no reason why the Spanish Flu had to remain particularly common. It could remain one of hundreds different viruses going around - flus, "common colds" and so on, and reach a few people here and there. But the particular 1918 strain was very visible - it killed a large percentage of people infected. If this virus was still extant, wouldn't we see small deadly flu epidemics in various places over the last 100 years? What in the flu virus or the circumstances allowed that not to happen? $\endgroup$ – Nadav Har'El Feb 16 at 21:56

It's didn't disappear. It's still around today, over 100 years later. The 1918 influenza virus is the parent virus for all the human seasonal influenza viruses that are around today, as well as for most of the swine influenza viruses out there.

In fact, the presently circulating H1N1 viruses in humans are close enough to the 1918 virus that they probably immunize against each other (Seasonal trivalent inactivated influenza vaccine protects against 1918 Spanish influenza virus infection in ferrets.).

What did happen is that the 1918 virus mutated, becoming less virulent in the process, within a year or so of entering the human population. Why? Presumably because, after infecting virtually the entire human population in its first year, there was so much population immunity that it needed to mutate a little in order to continue to infect. That is, after all, what flu viruses do.

But it continued to circulate for decades, as a very similar H1N1 virus. Also, it almost immediately infected pigs and became endemic in the swine population, only to resurface after 90 years to cause the 2009 influenza pandemic.

In 2009, persons who had lived through the first decades of the 1918 pandemic H1N1 era, especially those born before about 1950, were substantially protected from the 2009 pandemic virus by having acquired immunity to the antigenically similar H1 or N1 of the 1918 virus, or both, or to the descendant seasonal H1N1 viruses that circulated over subsequent decades. This is because the 2009 H1 gene was a 1918 viral progeny that had survived for more than 90 years, with minimal antigenic drift, in a domestic pig “time capsule.”

The Mother of All Pandemics Is 100 Years Old (and Going Strong)! (Am J Public Health. 2018 November; 108(11): 1449–1454)

In 1957 and 1968, the H1N1 virus reassorted with avian viruses to generate first H2N2 and then H3N2 viruses (the great-grandchildren of the H3N2 pandemic still circulate today). But most of the internal genes in those reassortants remained the 1918 versions.

So the 1918 flu never went away. It’s still killing hundreds of thousands of people every year. It’s just that everyone has got used to it, and thinks it’s fine and normal to have hundreds of millions of influenza cases every year.

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  • $\begingroup$ Thanks for the detailed and informative response! The reason why I'm still puzzled by all of this is that the 1918 strain was distinctive in that it killed a very large percentage of young, otherwise-healthy, of infected individuals. Somehow we're not seeing this phenomenon any more. Even if the original strain mutated to create 100 different strains, if the original strain was still around as well, wouldn't we have seen small local epidemics of it from time to time killing large percentages of the healthy adults in certain towns, etc.? $\endgroup$ – Nadav Har'El Feb 18 at 18:07
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    $\begingroup$ We don't know in detail why the 1918 flu was so virulent; we do know that the surface protein hemagglutinin (HA) was a significant part of it, and we know that the HA of flu viruses changes very rapidly in response to population immunity pressure. The presumption is that virulence depended on a very narrow and precise sequence, that changed as a result of adaptation to human immunity - probably because virulence per se was not an adaptive trait but was simply a side effect of other factors. $\endgroup$ – iayork Feb 18 at 19:28
  • $\begingroup$ nice answer! Another related question: Before 1918, there were other influenza viruses circulating? The 1918 influenza was of a zoonotic origin? Where did it came from? $\endgroup$ – juan Isaza Mar 26 at 4:44
  • $\begingroup$ 1918 was an avian influenza, perhaps passed through swine before infecting humans. Many strains of influenza circulated before 1918, but we don’t know much about them because we don’t have samples. There were many pandemics, presumably zoonotic, most probably avian origin but perhaps also equine $\endgroup$ – iayork Jul 3 at 15:54
  • $\begingroup$ Check out the below link where I have put a graph on why it killed more young people. I have also linked there a PNAS article if you're curious. biology.stackexchange.com/questions/95652/… $\endgroup$ – m4rio Sep 11 at 3:48

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