Infectious mononucleosis is an illness caused by a viral infection. It is commonly called mononucleosis, or "mono." Mononucleosis is most often caused by the Epstein-Barr virus. In some cases, it is caused by other viruses.
Mononucleosis has been nicknamed the "kissing disease." This is because Epstein-Barr virus commonly is transmitted during kissing. However, sneezes and coughs also can transmit the virus.
Mononucleosis typically occurs the first time a person is infected with Epstein-Barr virus. But infection with Epstein-Barr virus does not always cause mononucleosis. It often causes only a mild illness or no illness at all.