Curious as to how -itis got to be the medical suffix meaning inflammation, as in "arthritis," joint inflammation, I investigated and found that -ῖτις, -ίτιδος is a femine adjectival form. Thus, noun ἄρθρον, joint, becomes ἀρθρῖτις, -ιδος, ἡ, arthritis, a shortened, nominal form of the original ἀρθρῖτις νόσος, joint disease. Originally, ἀρθρῖτις just meant of or pertaining to joints, not necessarily implying disease. It was a femine adjective for νόσος, disease, and over time the νόσος was commonly dropped.
The weird thing is that I don't know of any other instances of this -ῖτις ending. The corresponding masculine form is supposedly -ίτης, -ου, but I can't find or think of any other examples of this either.
I can't find anything about this in Smythe or the Cambridge Grammar. The best source I've found is the OED, which gives a nice explanation and further examples: νεφρῖτις, πλευρῖτις, ῥαχῖτις. I can't confirm ῥαχῖτις, but Aristotle is cited for the masculine ῥαχίτης. The only other -ίτης adjective I've found is πλευρίτης.
Does anyone know more about this? Are these endings just so obscure that the grammars don't bother with them?