Hmm, that's an interesting question. I'm not an evolutionary biologist but I am a biologist (more specifically, a microbiogist).

The crux of the misunderstanding, I think, is that the definition of what counts as advantageous or "good" has changed over time. Very rapidly, in fact. The reason many diseases are still around today is because many genetic diseases offered a very real advantage in the past. The example that is often given is malaria and sickle cell anemia. Sickle cell anemia gives resistance to malaria, which is why it's so prevalent in populations that historically have high incidence of malaria.

Natural selection doesn't improve anything, it just makes animals more fit for their exact, immediate situation. That also means that it is very possible (and in fact, very likely) that the traits that we today associate with health will become disadvantageous in the future.

If we remember that natural selection isn't trying to push humanity towards any goal, enlightenment, or good health, it becomes easier to acknowledge and accept that we can and should interfere with natural selection