[publius911]

If it's undiagnosed, how do they know it's rare?

The world hates a wise-ass.

Assuming you're serious, it must be rare because they CAN'T diagnose it.

[FoxInSocks]

The article didn't say they "CAN'T" diagnose the illness -- only that it hasn't been diagnosed. If you don't know what it is, you cannot say that it's rare.

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.