He’s not just being polite but he made the conscious decision that this store is the one he wants to support because they were willing to lose revenue in order to help the customer.

Well…they 'how’l all the fricken time without being asked!

But also is cross-immunisation. So…one could have had something other than Covid-19 and still be immune to it. Then there are also the genetic outliers that are just naturally immune to the attack-vector of the virus.