Interesting stuff

It causes covid-19 because that is what the disease was named when it was discovered to be novel, or new.

Sarduakar said:

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Herd immunity doesnt mean you dont get it ever. It means the transmission and viral load is lower. And that the average person has probably already had it and will thus have built up some resistance to it. But not an immunity where they would never get it.

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I'm no immunologist, but I think it really just comes down to math and rate of spread. In the beginning of an epidemic an infected person comes in contact with many people that don't have immunity and the infection spreads very quickly, growing exponentially. As more people get it and are immune, an infected person comes in contact with much fewer people that they can transmit it to and eventually the growth rate flattens and drops to some very low level. If you have not yet had it or have immunity you can still get it, but you're less likely to pass it on if you don't come in contact with someone that is still susceptible.