Epstein-Barr virus (EBV), a widespread human herpes virus, can cause mononucleosis — but usually it doesn't. In fact, most EBV infections aren't noticeable, even when they're most active in your body. By age 35, almost everyone has antibodies to EBV, indicating past infection.

It takes more than an uncovered cough or sneeze to transmit EBV. During primary infection, people shed the virus in saliva. You need close contact, such as kissing or sharing a cup with an infected person, to catch EBV.

The infection generally causes no signs or symptoms, except in teenagers and young adults. In that age group, at least a quarter of infections cause mononucleosis — a disease that features fatigue, headache, fever, sore throat and swollen lymph nodes.