This is not true at all. Long covid, chronic fatigue, etc. are currently not understood, and have no known mechanisms or tests. There is nothing for doctors to identify as 'wrong' besides the subjective feeling of fatigue. This is not a matter of doctors making mistakes or not, this simply is beyond the limits of our current medical understanding. It definately sounds like the parent is suffering from long covid.

Doctors all too often think that if you are experiencing something that their measurements don't validate ... that you are not sick.

In my experience [6 weeks hospitalization; now chronic rare disease] it's that the doctors simply lack the knowledge, techniques, and instruments to identify the problem.

Rare diseases are an interesting thing. Individually, each rare disease is challenging to study, and hence poorly understood, hard to diagnose, etc. However, with thousands to choose from, getting a rare disease isn't all that rare. However, getting good care while having one might very well be.

Nothing is wrong based on given measurements taken, their limits these days, skill set of doctor evaluating them and so on. If your lung capacity or strength of some particular heart muscle degraded by 7.3% and your veins became 2.3% more susceptible to popping nobody will find it out.

My wife is a doctor, worked on emergencies too, and the amount of "we have no clue what is wrong with him" cases is staggering. Sure they can more or less treat symptoms but often that's it.

People often make a mistake (I used to make it too) and take medicine/healthcare as some rigid science keen to astronomy or mathematics where numbers don't lie and you always know where you are and why. Couldn't be further from truth these days.

Just a random sample from me personally - had broken my foot this June, and had plaster for a month. Have taken anti-coagulant medicine during it. Stopped when plaster went away as per doctor, within few days I've developed an ache in middle of calf, it was unlike any tiredness I had ever experienced there. But that fixation made my leg stiff like wooden plank, so I thought its just super tiredness of muscles when attempting to walk again. Wife mentioned that maaaaaybe it could be thrombosis, who knows. Went to hospital, they did blood tests which young doctor concluded were below threshold so I should stop taking medicine immediately and all is a-ok. Well, luckily I also went to venereologist who did ultrasound and voila - trombosis like hell there.

I could have died anytime during this, when it gets to lungs its rather quick process, brain is no fun neither. All this in biggest hospital in Switzerland.

Generally trust doctors and science. Don't trust them blindly. I guess most folks have some stories like this, albeit I hope with less risks.