OK - I can see where you're coming from but these kind of things DO happen to occur at certain era. For example - in old time... prostrate cancer is UNHEARD of. Now prostate cancer is commonsome. why? because of advanced medical technology and improved sanitation/hygiene practice... our lives have been extended far longer than ever. Because of that - many new diseases and cancers have popped up.
Now for bipolar that just came out of nowhere in 1950... because we don't know for sure... I'm just pulling it out of my ass - maybe it's because of aggressive progression of technology that had unintended side effects on us... for ie - massive electrical field (computers, tv, radios, cellphones, ipod, etc). thousand of nuclear power plants. ubiquitous use of drugs - hundred different traces of medications are even in your tap water but at very harmless level.
scientists have a very strict, thorough methodology and system to categorize condition/disease/illness/etc. and that takes time. because of this thorough practice - million of lives have been saved. if we go by dreama's way... there's no doubt that it would create gigantic, disorganized mess and we would not be at this level of medical advancement because you'd have several names for same illness.