infirmity vs unfitness

infirmity and unfitness both are nouns.

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

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

infirmity: the state of being weak in health or body (especially from old age)
unfitness: poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)

Other hypernyms of infirmity include softness.

infirmity (noun)	unfitness (noun)
the state of being weak in health or body (especially from old age)	the quality of not being suitable
	lacking the power to perform
	poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)