Clarke , quoted Von Grauvogl , in his book "Constitutional Medicine" regarding the relative strengths of the immune system. Susceptibilty is high in some snd low in others .
In relation to sexually transmitted disease he said, "From the same woman on the same day - in deed in the very same hour , one may get a syphilitic ulcer , one a sycotic ulcer and one escape without any infection.
The point is that some may be carried off by minor infections, some fight off serious complaints and some never be ill.
To produce a person who can escape most or all ills it is necessary to treat homeopathically over some generations . Which is why the English royal family -- who have been treated for generations - should be public record!.
One of the problems of today is that there has been so much pharmaceutical medication inflicted on the human race that it is much weaker on the resistance front , than it should be. Antibiotics, Vaccines , Tranquillisers , Pain Killers , all weaken the immunology of the race .
However there will always be that 5% of the race that is immune to particular conditions - even AIDS !.
So while as individuals we may fear extiction , as a race we may be confident of survival.