The disease often referred to as the "kissing disease" is infectious mononucleosis, commonly caused by the Epstein-Barr virus (EBV). It spreads primarily through saliva, which is why it's associated with kissing but can also be transmitted through other forms of close contact. Symptoms may include fatigue, fever, sore throat, and swollen lymph nodes. Most commonly affecting teenagers and young adults, it typically resolves on its own with rest and hydration.