unfitness vs unhealthiness

unfitness and unhealthiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
unfitness	Yes	No	No	No
unhealthiness	Yes	No	No	No

As nouns, unhealthiness is a hypernym of unfitness; that is, unhealthiness is a word with a broader meaning than unfitness:

unfitness: poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)
unhealthiness: a state in which you are unable to function normally and without pain

Other hypernyms of unfitness include health problem, ill health.

unfitness (noun)	unhealthiness (noun)
the quality of not being suitable	a state in which you are unable to function normally and without pain
lacking the power to perform
poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)