According to The Nemours Foundation, people infected with the Epstein-Barr virus (EBV) that causes mononucleosis can spread the virus for up to 18 months after symptoms disappear. The exact period of time is not known, but the virus eventually becomes dormant, causing neither symptoms nor contagion.
The Nemours Foundation indicates that the virus spreads through saliva, which means that kissing an infected person or sharing drinks or utensils with her can cause EBV infection for as long as the virus remains active in the body. Mononucleosis symptoms only appear after an incubation period of around four to seven weeks. It is also possible to get infected with EBV but never have mononucleosis symptoms.