Hmmm, I think a grammar expert might have a more accurate answer, but here's how I see it. HIV is a noun, "HIV-related" (as a phrase) an adjective, and "mononeuritis multiplex" is a noun. I think we use this kind of device in many other instances, sometimes hyphenated, other times not, e.g., senior-adjusted price work-related injury form-fitted clothing, etc. In all of these cases, the hyphenated description (as a unit) functions as an adjective.