Thanks for your response, plotbot2015. The problem with your HIV comparison is that, unlike schizophrenia, there are laboratory tests to determine whether someone is afflicted by HIV. Before you can study the etiology of a particular ailment, you have to demonstrate that it is both valid and can be reliably diagnosed. Neither of which is the case for schizophrenia.
Yes, you are correct. At this moment in time, there's no test. In the early 1980s, there was no test for HIV, either. That's why they called it a syndrome.
https://www.aids.gov/hiv-aids-basics/hiv-aids-101/aids-timeline/
But we'll get there, eventually. I realize that is not much comfort to the people suffering now.