Mono is most common in people between 15 and 25, but is in no way limited to lip locking. In addition to saliva, the virus can be spread through mucus from the nose or throat and sometimes even tears. Saliva can also be carried through coughs (especially when your mouth isn't covered) or sharing toothbrushes, food, cups, utensils and lip makeup (lip liner, lip gloss, lipstick, etc...).

The Epstein-Barr virus (EBV) is very common and most kids are exposed to it at some point while growing up. The reasons are still unclear, but infants and younger kids infected with EBV usually do not show symptoms, while teens tend to develop mono.