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.,
form-fitted clothing, etc.
In all of these cases, the hyphenated description (as a unit) functions as an adjective.