Mononucleosis. Mononucleosis is incredibly common and exceptionally easily transmitted from person to person regardless of whether or not they appear "clean" and asymptomatic. Thus, any time you have mouth to mouth contact with another person you have a significant opportunity to contract mononucleosis as well as many other orally/mucosal surface transmitted diseases.
It happens. Many have mono as a kid when it is mild and similar to other viral illnesses. Once they do they are immune for life. As an adult, you cannot catch it unless you are not immune. Anyone can shed this virus off & on indefinitely after their illness, so any contact you have with saliva can get you sick. Direct, with a kiss or indirect by touching a shopping cart & touching your lips.