Notes about complaint are frigid (5,4)
I believe the answer is:
stone cold
'frigid' is the definition.
(I've seen this before)
'notes about complaint' is the wordplay.
'about' indicates anagramming the letters.
'complaint' becomes 'cold' (cold is a kind of disease**).
'notes' is an anagram of 'stone'.
'stone'+'cold'='STONE-COLD'
'are' is the link.
(Other definitions for stone cold that I've seen before include "Freezing" , "Frigid" , "Absolutely without heat" , "bitter" , "severely lacking in warmth" .)