The medical term for the kissing disease is mononucleosis, aka mono. It is a virus you can pick up from saliva when having a serious smooch with an infected person. You can also get it in other ways like sharing food, drinks, and using improperly washed glasses and eating utensils of someone who is infected. This contagious virus falls under the herpes umbrella.
The symptoms can vary between being severe and mild, and severe symptoms have the potential to impact on your daily for up to a few weeks at a time.