How is it possible that 3 US hospitals can not diagnose a previously healthy 18 year old girl who is now so ill that she is fighting for her life?

Is this common?
Is it uncommon?

I have to admit that I didn’t know such a thing really happened anymore. Particularly considering that one of the hospitals is supposed to be one of the best in the country, specifically regarding complex cases. How can so many doctors not know what is wrong with this girl?