Though mononucleosis is known as the "kissing disease", sexual intercourse may increase the odds of contracting the virus, a UK study suggests.
The findings, say researchers, could have implications for the vaccines now being tested against Epstein-Barr virus (EBV), the cause of mono.
Their study found that of 510 college students who were initially free of EBV, nearly half became infected with the virus over the next three years, with sexually active students being at greater risk.
Students who said they'd had intercourse during the study were more likely to become infected than those who reported no romantic relationships, as well as those whose relationships were limited to kissing and petting.
The findings suggest that sexual intercourse itself makes EBV transmission more likely, according to the study authors, led by Dr Dorothy H. Crawford of the University of Edinburgh, who report their findings in the journal Clinical Infectious Diseases.
Most adults worldwide are infected with EBV, which, after first infecting a person, remains dormant in the body for life. Most of the time, the initial infection causes no apparent symptoms, especially when it's contracted in childhood.
In developed countries, however, EBV infection often doesn't occur until adolescence or young adulthood, when it's more likely to cause symptomatic illness, mono a condition marked by fever, fatigue, sore throat and swollen lymph nodes.
According to Crawford's team, the new findings suggest that sexually active teens and adults may be exposed to a larger dose of EBV through particularly "deep" kissing, or possibly through genital fluids, which can carry the virus.
If a large "viral load" increases the odds of developing mono, the researchers note, then an EBV vaccine may be successful even if it merely decreases the amount of EBV in the body rather than providing complete immunity.