declining health vs sickness

declining health

sickness

noun
  • The analogical misuse of a rarer or marked grammatical case in the place of a more common or unmarked case. 

  • The quality or state of being sick or diseased; illness. 

  • Nausea; qualmishness; as, sickness of stomach. 

How often have the words declining health and sickness occurred in a corpus of books? (source: Google Ngram Viewer )