word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.
<a href="https://www.etymonline.com/word/tonsillitis">Etymology of tonsillitis by etymonline</a>
Harper, D. (n.d.). Etymology of tonsillitis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/tonsillitis