I think it takes significant mutation before the immune system does not recognize a virus it has previously encountered and defeated.
I would research it for yourself, but I think it’s a REALLY significant mutation.
It irritates me that I can’t count on our scientists and doctors to tell me the truth anymore and I have to delve into their profession to try and divine the truth.
Build up the immune system.. isn’t that significant on why some lived thru civid and some died.
A weak immune, and a virus that attacks the immune.. no resistance..or not enough.
To build. . Heavy doses of all the vitamins plus co enzyme 10.. (cq10) and royal jelly.
For starters.
The Gain of Function bioweapons engineers use humanized mice/ferrets and similar creatures to perform "serial passage". Directed evolution takes place in an animal with a short life span and human receptors. The serial passage continues with the engineers selecting for infectivity and related pathogenesis. Normally this kind of "research" is conducted in BSL3 or BSL4 labs so it never gets outside of the lab...unless intentionally released.