Mono like the cold or flu can be transmitted through kissing, sneezing, coughing, etc. However, unlike the cold or flu, the person who transmits the virus [either EB or CMV] often doesn't have any symptoms. Both viruses are very common [50%+ of the population] but only a very small percent of people infected ever show symptoms. These viruses can lie dormant for years in an infected person and make their appearance years later.
In other words, for all you know, you might have infected your girlfriend or she might have been infected when she was a toddler but the symptoms only appeared recently because her immune system happens to be stressed at this point in time.
Here's what the Center for Disease Control [CDC] says about EBV mono - the most common mono:
[This message has been edited by Jay Tor (edited 09-14-2002).]