system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why did the 1918 flu pandemic stop? If that same flu came now, would it be a major pandemic again?

What was the 1919 flu season like?
gpt
The 1918-1919 influenza pandemic was so bad because it was an introduction of a brand new type of flu - H1N1.  No (or very few) living people at that time had any previous exposure or immunity to H1N1.  Thus, when the influenza virus shifted to this new type, nobody's immune system had any defense.  Since that time, H1N1 has drifted a little each year, but not enough to cause a pandemic quite like the one in 1918.  The virus did shift other times, and H2N2 and H3N2 types were introduced, causing their own pandemics.  This seems to happen every 60 years or so, as people who were exposed an children begin to pass away and what is called "herd immunity" wanes.

We had another, milder H1N1 pandemic introduced in 2009.  That virus still circulates today and causes most of the human flu cases each year.  If the 1918 flu strain was reintroduced, it is very likely that it would not cause nearly the death toll it did back then, due to the fact that the closely related 2009 H1N1 gives us some protection.