The Epstein-Barr virus infects almost everyone by the age of 40 and usually does not cause any illness. But it can cause mononucleosis (mono) and is linked to some types of cancer, such as Burkitt's lymphoma and cancers of the mouth.
The virus lives in a person's salivary glands and can be spread through saliva, such as by kissing or sharing drinking glasses or eating utensils.