Mono, short for infectious mononucleosis, is a disease caused by the Epstein-Barr virus (EBV). Mono is sometimes called the “kissing disease” because it is primarily spread through saliva but it can sometimes pass from person-to-person though coughing and sneezing. It is most common in adolescents and young adults and causes extreme fatigue, high fever, muscle aches, and a very painful sore throat that may last for weeks or even months.

Infection with EBV is very common and does not always lead to mono. People who are exposed to EBV as young children usually have no symptoms at all, or else their symptoms are indistinguishable from other mild childhood illnesses. Between 30% and 50% of people who are not exposed to EBV until adolescence or young adulthood experience the symptoms of mono when they do become exposed, while the rest experience more mild symptoms or no symptoms at all. Mono in older adulthood is rare, as an estimated 95% of Americans have been exposed to EBV by the age of 35. In less developed nations, most people are exposed to EBV at a very young age.

After being infected with EBV, regardless of how old a person is at the time of infection or whether or not they experience the symptoms of mono, everyone produces antibodies against EBV and becomes immune to mono. However, the virus remains in the body for life and may be passed on to others. In a small percentage of people, the virus may re-awaken and cause mono symptoms later in life.