Can we say with 100% certainty that for all viruses some portion of humans are immune. Nope. What the probability is we don't know. That's my only point, we don't know for sure.
I would proceed as if no one was immune, until I knew for certain who was or was not. Just to have an abundance of caution.
Some people do survive, that we know. What I haven't seen is whether that has been due to some “natural immunity” or whether those people just have super immune system responses to viruses.
Are you aware of any scientific studies investigating the issue, or are you just asserting that There are so many humans around that “a certain percentage of us, somewhere, must have immunity to any particular virus”?
If so, I would love to have a link or list so that I could read up on it. I would be very useful knowledge and a very interesting thing to read about I think.
You said: "Some people do survive, that we know. What I haven't seen is whether that has been due to some “natural immunity” or whether those people just have super immune system responses to viruses."
So, I guess we have to define "immunity". If you define immunity as "..people do survive" then the immunity rate is as high as 50% and as low as 10% for some strains. That we know too.