Like the CCR5-Delta32 mutation makes 10% of Europeans resistant to HIV (https://www.sciencedaily.com/releases/2005/03/050325234239.h...).

There are other mutations that appears to combat virus more effectively, but who's to say that there are not mutations that are beneficial for some disease, but catastrophic for others ?

but you can easily have regular checkups and still have an undiscovered (unknown) pre-existing condition. so what is the difference?

I think most people are immediately thinking severe asthma, COPD, recent or current cancer treatment; and while all of those elevate your risk, they aren't the most common comorbidities.

The key co-morbidities are high blood pressure, heart disease and diabetes.

Only heart disease seems obvious - high blood pressure is almost standard, and diabetes is often managed well to the point of invisibility.

The cardiac link appears interesting. Patients who are given ECMO, a complete lung bypass alternative to ventilators, have still died after their lungs recovered.