What is so peciuliar about bipolar?

Is it a continuum (mood episodes grading, severity, frequency, mixture of symptoms in same episode, many MDD shift bipolar in long term [first episode is depressive], residual manic episodes predict depressive recurrence, bipolar signs may be common in MDD makes difficult dx) So we have phenotype and its opposite and middle, diverse cluster of symptoms, self-lethality, multiple comorbidities, AD may be saviours or killers.