It has to do with RNA sequencing.... I.e. genetics.
Seasonal flu is an “all human virus”. The DNA/RNA chains that make up the virus are recognized by the human immune system. This means that your body has some immunity to it before it comes around each year... you get immunity two ways...through exposure to a virus, or by getting a flu shot.
Novel viruses, come from animals.... the WHO tracks novel viruses in animals, (sometimes for years watching for mutations). Usually these viruses only transfer from animal to animal (pigs in the case of H1N1) (birds in the case of the Spanish flu). But once, one of these animal viruses mutates, and starts to transfer from animals to humans... then it’s a problem, Why? Because we have no natural or acquired immunity.. the RNA sequencing of the genes inside the virus isn’t human, and the human immune system doesn’t recognize it so, we can’t fight it off.
Now.... sometimes, the mutation only allows transfer from animal to human, for years it’s only transmission is from an infected animal to a human before it finally mutates so that it can now transfer human to human... once that happens..we have a new contagion phase. And depending on the fashion of this new mutation, thats what decides how contagious, or how deadly it’s gonna be..
H1N1 was deadly....but it did not mutate in a way that was as deadly as the Spanish flu. It’s RNA was slower to mutate and it attacked its host differently, too.
Fast forward.
Now, here comes this Coronavirus... it existed in animals only, for nobody knows how long...but one day, at an animal market, in Wuhan China, in December 2019, it mutated and made the jump from animal to people. At first, only animals could give it to a person... But here is the scary part.... in just TWO WEEKS it mutated again and gained the ability to jump from human to human. Scientists call this quick ability, “slippery”
This Coronavirus, not being in any form a “human” virus (whereas we would all have some natural or acquired immunity). Took off like a rocket. And this was because, Humans have no known immunity...doctors have no known medicines for it.
And it just so happens that this particular mutated animal virus, changed itself in such a way the way that it causes great damage to human lungs..