BACKGROUND: Infection with Epstein-Barr virus (EBV) is almost ubiquitous in 
humans and generally occurs at two ages: infantile, which is usually 
asymptomatic and associated with poorer socioeconomic conditions, and 
adolescent, which causes infectious mononucleosis (IM) in ~25% cases. The 
determinants of whether the infection causes IM remain uncertain. We aimed to 
evaluate seasonality and temporal trends in IM.
METHODS: Data from all Monospot tests, used as a marker for IM, were collected 
from the Grampian population over 16 years.
RESULTS: Positive Monospot test results peaked at 17 years in females and 19 in 
males. Females had 16% more diagnoses, although 55% more tests. IM was ~38% more 
common in winter than summer. The annual rate of positive tests decreased 
progressively over the study period, from 174/100 000 (95% CI 171-178) in 1997 
to 67/100 000 (95% CI 65-69) in 2012.
CONCLUSIONS: IM appears to be decreasing in incidence, which may be caused by 
changing environmental influences on immune systems. One such factor may be 
exposure to sunlight.Words 168.
FUNDING: The Medical Research Council and NHS Grampian-MS endowments.