I assume you're talking about Mononucleosis, "Mono" for short. It's a sickness, and it's contagious. They often call it "the kissing bug". It's commonly transfered through kissing, sharing drinks and any other way of swapping saliva. You can read more about it online. Here is a great website to look over:
http://www.faqs.org/health/Sick-V3/Infectious-Mononucleosis.html