Im no doctor, just an ap bio hs student (take that as you will) but from what I've read, the COVID-19 is an ssRNA virus, meaning that it has the potential to mutate VERY easily. I might be wrong, but I've read that bc of its outermost "shell" being, in part, of a similar composition to that of our own respiratory cells, it is extremely difficult for our body to detect it and do something about it before the virus hijacks the cell and uses its machinery to mass-produce hundreds of itself.

I'm not too sure about the specifics of COVID in the context of its ability to come back and for ppl to get it twice, but it might undergo a lysogenic cycle, in which after you are infected with the virus, it "fuses" its DNA with your own, laying dormant until later in life, when your immune system is weakened, to then reemerge as another issue, kind of like how ppl who have gotten chicken pox as children are at a high risk of developing shingles when they're adults.

I skipped over a lot of stuff, and I also dont know enough about a lot of stuff, so take all of this with a grain of salt. Also, again, I am in no way qualified to 100% say that all of this is 100% applicable to what we're seeing with COVID, so pls correct me bc I know I prolly made some mistakes lol (I take the class, but my grades are questionable.. :D)