Nazo no Kanojo X, Sakuranbo Syndrome - Cupid no Itazura II Recommendations
Both of these titles have underlying supernatural circumstances that push characters to swap saliva.
If 'strange' is a word on your vocabulary list when you're trying to find a series, then these two titles should definitely be on the look out for. Both series involves a female character with abnormal condition/habit. The relationship they develop with the main male protagonist puts them on oddball circumstances that often result in awkward moments. Expect peculiar comedy and plot twists that unravel themselves as the story progresses.