If you’re going to talk “survival of the fittest” related to a virus, understand what that means. The term “fit” in survival of the fittest does not refer to being physically fit or strong; it refers to “fit” as in how a foot fits a shoe. The usage of fit to refer to physical fitness or strength is a newer usage of the word. In ‘On the Origin of Species’, Darwin introduced the phrase in the fifth edition published in 1869, intending it to mean “better designed for an immediate, local environment“.
To go further, it’s not like one person better fits the environment with the virus in it and lives while one person doesn’t and dies. It’s not a fight between a virus and a human. The question of survival of the fittest is between the virus and the inner environment of the person infected. No virus ever killed anyone; it just survived in the environment of the human long enough to replicate itself a lot and weaken the host to the point of death. Without knowing how the virus might mutate or what helps it survive inside a person; herd immunity is a fool’s game.
“Natural selection” has no bearing on the survival of any individual human. This is true for two reasons. First, natural selection was never about any individual organism surviving, it’s about the mutations (or changes) necessary to help a whole species adapt to its environment over a long period of time. Second, human beings, specifically, don’t live within nature anymore, so humans are subject to “societal selection” — humans now largely decide what traits to carry on into the future, not nature.
Natural selection may become the downfall of the human species and lead to the destruction of society. The chemicals that produce a fight, flight, or flee response in nature; the instincts to fight for food and to feel dominance over other animals and nature; these things are detrimental now. After societies developed, the human animal participated in this “fight” for survival less and less. The feelings of dominance over nature got transferred from hunting to dominance over other people and other societies, becoming self-destructive war-making and violence toward other humans.
Even the mass destruction of nature for development may be driven by naturally selected ‘homemaking’ behaviors run amok using machinery. Unless humans consciously evolve to limit the controls these chemicals have on behaviors, human society may very well destroy the environment to the point where humans and many other mammals won’t be a “fit” to survive on Earth anymore.