If your friend has been diagnosed with mono, also known as infectious mononucleosis, this is caused by a virus called Epstein-Barr virus. This infection can affect the spleen, liver and cause extreme fatigue. If he has not been tested for HIV, then I would recommend getting this test because initial HIV infection can also look like mono. He will hopefully begin to feel better in a few weeks, some patients can take up to 5-6 months to feel back to normal. The treatment for this is supportive care which means rest and plenty of fluids. You asked a very good question of how long he will be contagious, this is unknown but is likely at least a few weeks to months. I would suggest not sharing food or drinks, kissing, or sharing personal items such as a toothbrush to your friend with anyone for at least 6 months. While his spleen is affected, I would also tell him to stay away from contact sports since sudden trauma can cause splenic rupture. Thanks, Dr. Patel