Just briefly glancing through here it seems like anywhere from 15-25% could be diagnosed as morbidly paranoid, with 5% of that as full bore schizophrenia.
Anyone having a bad day/week/month can get labelled with alsorts of disorders. The field of psychology isn't very advanced yet, the brain is highly complex.
Sure, it is advanced. They developed so many meds, that will fuck up your brain chemistry or your hormons. Anything else just isn't that important.