While usually caused by Epstein–Barr virus (EBV), also known as human herpesvirus 4, which is a member of the herpes virus 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. 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. Prevention is by not sharing personal items with or kissing those infected. Mono generally gets better on its own. Recommendations include drinking enough fluids, getting sufficient rest, and taking pain medications such as paracetamol (acetaminophen) and ibuprofen.
Mono 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 the symptoms are less. 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".
Before puberty, the disease typically only produces flu-like symptoms, if any at all. When found, symptoms tend to be similar to those of common throat infections (mild pharyngitis, with or without tonsillitis).
Another major symptom is feeling tired. Headaches are common, and abdominal pains with nausea or vomiting sometimes also occur. Symptoms most often disappear after about 2–4 weeks. However, fatigue and a general feeling of being unwell (malaise) may sometimes last for months. Fatigue lasts more than one month in an estimated 28% of cases. Mild fever, swollen neck glands and body aches may also persist beyond 4 weeks. Most people are able to resume their usual activities within 2–3 months.