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.
EBV. Mono is short for infectious mononucleosis. It is caused by the epstein-barr virus or ebv.