The world hates a wise-ass.
Assuming you're serious, it must be rare because they CAN'T diagnose it.
If you are awaiting lab results to confirm your pneumonia, you can say your illness is undiagnosed -- but that doesn't mean pneumonia is rare. It also doesn't mean that pneumonia can't be diagnosed.
We're pointing out sensationalism and inaccurate "reporting." There is very little of substance provided in the story.