1650s, "of the nature of a disease, indicative of a disease," from Latin morbidus "diseased," from morbus "sickness, disease, ailment, illness," according to de Vaan perhaps connected to the root of mori "to die," as "looking like death" (from PIE root *mer- "to rub away, harm," also "to die" and forming words referring to death and to beings subject to death), or from a non-IE word. Meaning "diseased, sickly" is from 1731. Transferred use, of mental states, "unwholesome, excessive, unreasonable" is by 1834. Related: Morbidly; morbidness. Middle English had morbous "diseased" (early 15c.), from Latin morbosus.
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]