[ShadowAce]

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

[FoxInSocks]

That was the question that popped into my head!

[Robert DeLong]

They know it’s rare because the symptom’s have never been documented before. In other words the symptom’s do not match any previously diagnosed cases.

[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.