Sex ratio of infectious mononucleosis and possible relevance to multiple sclerosis.

Infectious mononucleosis (IM) is associated with the risk of developing multiple sclerosis (MS). Using databases of hospital admissions for England (1999-2005), we investigated the female-to-male ratios (FMRs) for admission to hospital for IM and MS stratified by age. Males were more frequently admitted for IM for all age groups apart from ages 10-14 (FMR 1.50; 95% confidence interval (CI) 1.36-1.64) and, borderline significantly, at ages 15-19 (FMR 1.03, 95% CI 0.99-1.08). This intriguing aspect of IM epidemiology in adolescence, the atypical female excess, may be linked to the sex ratio of MS, where females predominate from adolescence.