(sorry in advance for the below TMI 
)
And COVID-19 uses those spikes in a wild way to attach (like grappling hooks) to what would become a host cell and then does all sorts of wild chemical reactions to burst into that host cell (which could be located in the lungs or on a kidney, etc) and reproduce. Then on its way out, the babies make it so that the body's defenses have a hard time getting to the infected cell to clear it. It literally does a "MacGyver" to a cell to get in, reproduce, and then get out. 
This is the image I continue to find wild (what I call "the grappling hooks" that pull it to the cell that it has attached to) -
This is what really differentiates it from the flu. With the variants of COVID changing what the spike looks like including "hiding" the changes from antibodies due to the new types of proteins that make up the spike, folding themselves over in a certain way to block those newly-changed segments from being accessed by antibodies, it makes it harder to fight.
And with respect to "the plague" - well that is caused by a bacteria and not a virus (bacteria are much bigger in size when compared to viruses), so once antibiotics were discovered and refined, that was pretty much the end of "the plague" (in humans)! That's why it stopped being "a threat" (as long as someone who is infected can get the antibiotics for it). I know it is still appearing in some areas like the west and southwest - including in places like CO or TX where there may be prairie dogs infected with it, and can pass it on.
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