Can a doctor to determine if someone is still contagious with mono?

Sometimes. The most infectious period of mononucleosis is within the first few weeks of the illness. As you feel better and your antibodies rise against the epstein barr virus (ebv), you become less contagious. However, ebv remains in the body for life. At times the virus may be shed when you are well and at times it can be passed on with a recurrence of illness. The contagious period is indefinite.