There are two groups of people whose case numbers saw a huge spike and then a sharp decline. Those two groups are:
1) Folks in cities very hard hit early on
2) Old people
The curves of both are strikingly similar.
Here are two possible explanations:
A) Herd immunity
B) Learned behavioral response
A) could conceivably explain what we are seeing with group 1), although that would be extremely unlikely. A) certainly does not explain what we are seeing with group 2).
B) could easily explains what we are seeing with group 1). A) also easily explains what we are seeing with group 2).
Conclusion: B) is the right answer.
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No need for you to be so sensitive to perceived sarcasm in other folks' posts. No sarcasm intended.
Asking the question was fine. I have provided you with the answer. If you don't want the answer, then don't ask the question.