Before we get overly freaked out, it’s inappropriate to multiply the entire population by the mortality rate. Many, usually most, people don’t actually get the disease, and the mortality rate is calculated from those who contract it.
The US population in 1918, during the Spanish Flu, was about 110M. About 28% contracted this disease, or about 30M. Of these, about 500,000 to 700,000 died, or about 2 to 3%.
The rate of contraction and the mortality rate varied in different parts of the world.
But even this disease, probably the most virulent in human history, killed only about .5% of the US population. Today that would be about 1.5M, a horrible tragedy but not nation-destroying levels. We are also much better equipped to fight a similar disease than in 1918. Not adequately, but better.
Until it decides to mutate from a bad bug to a super bad bug and we don't any way to stop it.
It would be life changing. EVERYONE back then kow people who died. Tey had to burn the bodies, shut down businesses, quarantine people. It could get bad very fast and I’m not just talking about health effects.