I think you are somewhat contagious to very contagious if you have the virus
or not contagious if you don't have the virus.

Kids often get the virus, but don't get symptomatic. So they never know they
have it or had it. It takes 4-6 weeks for symptoms to show up. It appears to
take a few weeks to become positive for the antibody tests, so you won't
know for sure that you didn't get the virus for several weeks, probably
about seven.

Two sources of information are wikipedia and the CDC.

http://www.cdc.gov/ncidod/diseases/ebv.htm
http://en.wikipedia.org/wiki/Mononucleosis

The CDC says this:  "Symptoms of infectious mononucleosis are fever, sore
throat, and swollen lymph glands. " and "Most individuals exposed to people
with infectious mononucleosis have previously been infected with EBV and are
not at risk for infectious mononucleosis. In addition, transmission of EBV
requires intimate contact with the saliva (found in the mouth) of an
infected person. Transmission of this virus through the air or blood does
not normally occur. The incubation period, or the time from infection to
appearance of symptoms, ranges from 4 to 6 weeks. Persons with infectious
mononucleosis may be able to spread the infection to others for a period of
weeks. However, no special precautions or isolation procedures are
recommended, since the virus is also found frequently in the saliva of
healthy people. In fact, many healthy people can carry and spread the virus
intermittently for life. These people are usually the primary reservoir for
person-to-person transmission. For this reason, transmission of the virus is
almost impossible to prevent."

Basically, if you have had the virus, you may be passing it on to others
forever, at least now and then.

I don't think you can do anything about it.

Jeff