Mononucleosis is highly contagious, and can be passed to another person through kissing, sharing drinks, etc. Mononucleosis also ranges in severity from person to person - some people only experience mild fatigue, where others exhibit severe symptoms that require hospitalization. Also, it may take several months from the date of exposure for it to develop into mono.
From my own experience...I came down with a moderate case of mono during finals week of my junior year of college in 2003. It started out as a sore throat and the sniffles...it turned into what I thought was the worst cold I'd ever had, followed by the worst flu I'd ever had. About three weeks after I first started showing symptoms, I had the blood test done and it showed up positive for mono. They suggested that I be hospitalized based on the severity of my symptoms, but I declined, given that I was in the middle of finals. It took me about two months to get completely over the symptoms. I ended up having reoccurences of mono in 2005 and 2006...both times, I started exhibiting symptoms immediately following an extended period of high stress (according to several doctors I have talked to, you can have mono more than once - contrary to popular belief).
As for giving it to others...the first time I had mono, I had shared drinks with several friends during the first few days of showing symptoms, and none of them developed mono, to my knowledge. However, when I had mono in 2006, a friend that I shared drinks with (I know, you'd think I would have learned my lesson...) a week prior to my symptoms showing up came down with mono within a month of me.
So, exposure to the mono virus does not automatically mean you will come down with it. The best way to prevent mono is take steps to protect yourself from exposure - do not kiss or share drinks with anyone who is showing signs or symptoms of illness, etc. If you do get it, it's not a death sentence, but you will need to take good care of yourself while you recover...make sure you get plenty of rest, drink lots of fluids, and refrain from exercise or strenuous physical activities during the duration of the illness (to prevent spleen rupture). There's no real medication for mono, but if you are having problems with your throat swelling/sore, your doctor may recommend a steriod treatment to reduce the swelling and soreness (this worked for my brother when he came down with mono).