I have seen the following post popping up on Facebook recently:

> 🧐🧐🧐
> 
> Feeling confused as to why Coronavirus is a bigger deal than Seasonal
> flu?  Here it is in a nutshell.   I hope this helps.  Feel free to
> share this to others who don’t understand...   
> 
> 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 can 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..
> 
> That’s why Coronavirus is different from seasonal flu, or H1N1 or any
> other type of influenza....  this one is slippery AF.   And it’s a
> lung eater...And, it’s already mutated AGAIN, so that we now have two
> strains to deal with, strain s, and strain L....which makes it twice
> as hard to develop a vaccine.
> 
> We really have no tools in our shed, with this.  History has shown
> that fast and immediate closings of public places has helped in the
> past pandemics.  Philadelphia and Baltimore were reluctant to close
> events in 1918 and they were the hardest hit in the US during the
> Spanish Flu. 
> 
> Factoid:  Henry VIII stayed in his room and allowed no one near him,
> till the Sweating Sickness that plagued his time had
> passed...(honestly...I understand him so much better now). Just like
> us, he had no tools in his shed, except social isolation...
> 
> And let me end by saying....right now it’s hitting older folks
> harder... but this genome is so slippery...if it mutates again (and it
> will). Who is to say, what it will do next.  
> 
> Be smart folks...  acting like you’re unafraid is so not sexy right
> now.  
> 
> \#flattenthecurve.  Stay home folks...  and share this to those that just are not catching on.  🤓

Most of this I have no particular dispute with, but the claim

>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. 

sounds like saying that we can only _gain_ an immunity to a virus that has human RNA in it. It seems to me more likely that we merely don't have any _existing_ immunity to it because it is brand new to the human immune system; now that we've seen it, we'll gain immunity to it, and who cares what RNA is inside it? (And again, my gut says that it is the human RNA that actually gives it access to infect humans and thus become prevalent enough in the body to become a valid target for the immune system). But I'm no virologist, and I'm curious what the reality is.