A virus infection. Mono is due to an infection by ebv virus, and is seen more often in teens and young adults. Ebv virus is contagious in the saliva. Younger kids who catch ebv usually do not get the mono syndrome of prolonged fatigue, aches, swollen lymph glands, huge tonsils, fevers, sore throat, swollen spleen, puffy face, etc... Patients get better after a couple weeks or longer.
Viral syndrome. Mono is short for mononucleosis, named for a type of white blood cell that show up in greater percentages during the illness. Most 85% is caused by the EB virus, while the rest include a handful of other viruses including CMV. Another common name is glandular fever, because of the prominent swelling of lymph glands and spleen.