Infectious mononucleosis (IM, mono), also known as glandular fever, is an infection usually caused by the Epstein–Barr virus (EBV). Most people are infected by the virus as children, when the disease produces few or no symptoms. In young adults, the disease often results in fever, sore throat, enlarged lymph nodes in the neck, and fatigue. Most people recover in two to four weeks; however, feeling tired may last for months. The liver or spleen may also become swollen, and in less than one percent of cases splenic rupture may occur.

While usually caused by the Epstein–Barr virus, also known as human herpesvirus 4, which is a member of the herpesvirus family, a few other viruses may also cause the disease. It is primarily spread through saliva but can rarely be spread through semen or blood. Spread may occur by objects such as drinking glasses or toothbrushes or through a cough or sneeze. Those who are infected can spread the disease weeks before symptoms develop. Mono is primarily diagnosed based on the symptoms and can be confirmed with blood tests for specific antibodies. Another typical finding is increased blood lymphocytes of which more than 10% are atypical. The monospot test is not recommended for general use due to poor accuracy.

There is no vaccine for EBV, though promising vaccine research results exist. Infection can be prevented by not sharing personal items or saliva with an infected person. Mono generally improves without any specific treatment. Symptoms may be reduced by drinking enough fluids, getting sufficient rest, and taking pain medications such as paracetamol (acetaminophen) and ibuprofen.

Mononucleosis most commonly affects those between the ages of 15 to 24 years in the developed world. In the developing world, people are more often infected in early childhood when there are fewer symptoms. In those between 16 and 20 it is the cause of about 8% of sore throats. About 45 out of 100,000 people develop infectious mono each year in the United States. Nearly 95% of people have had an EBV infection by the time they are adults. The disease occurs equally at all times of the year. Mononucleosis was first described in the 1920s and is colloquially known as "the kissing disease".