Cordyceps species are known to produce numerous active components and are used for diverse medicinal purposes because of their varied physiological activities, including their anticancer, anti-inflammatory, and antimicrobial effects. Cordycepin, one of adenosine analogues, differs from adenosine in that its ribose lacks an oxygen atom at the 3' position. Cordycepin has been reported to be a main effector molecule in Cordyceps extracts that executes antiviral activities against several viruses including influenza virus, plant viruses, human immunodeficiency virus (HIV), murine leukemia virus, and Epstein-Barr virus (EBV). In addition, adenosine and its deaminase inhibitor showed strong antiviral potentials that were about 4,000 times more potent than the activity of the direct inhibitory effect by adenine arabinoside on Herpes simplex virus (HSV)-1, suggesting adenosine itself plays an important role to produce antiviral activities. In this study, we show that cordycepin and adenosine possess antiviral and antitumor activities against EBV and EBVassociated gastric carcinoma, respectively. Furthermore, we report epigenetic mechanisms used by cordycepin and adenosine to exert their antiviral and antitumor effects.