The 19th century, German philosophy, Friedrich Nietzsche once said “that which doesn't kill us makes us stronger,. I never quite agreed with that and if we look at the
graph of antifragility, we can see why. (That which doesn't kill can sometimes make us permanently weakened!)
Eustress, which is the form/quality/quantity of stress that can make us stronger, has a limit. Too much stress turns into distress, which is harmful or even deadly. So, for some people the coronavirus will indeed make their immune system stronger: that would be eustress. But, for others, the virus will be too much and the distress will kill them, or leave them with severely weakened lungs and breathing problems. The theory of antifragility postulates that we need stress to regain and maintain optimal healthy but too much stress is unhealthy. (Also, no stress at all is unhealthy!) Where the balance between too much and too little lies varies considerably by one's biography and biology.