- Recent evidence that infectious mononucleosis (IM) may be sexually transmitted prompted the present analysis. Infectious mononucleosis is a notifiable disease in the Israel Defense Forces (IDF). For the present study, the archives of the IDF were reviewed for all cases of IM from January 1, 1978 to December 31, 2009, and the rates were calculated. Annual rates decreased from 2.99 cases per 1,000 in 1979 to a low of 0.38 cases per 1,000 young adults in 1987. Between 2002 and 2009, the average annual rate was 0.88 cases per 1,000, just half the average rate of 1.69 observed between 1989 and 2001. Average monthly rates varied from a low of 0.90 cases per 10,000 in February to a high of 1.50 cases per 10,000 in August. The difference in the average rates between winter (1.02 cases per 10,000 soldiers) and summer (1.29 cases per 10,000 soldiers) was significant (p < 0.01). Analysis of the long-term epidemiology of IM shows that the infection rate has varied over time, and that the disease is more prevalent in the warmer months. This seasonality trend was also observed in several STD, raising the possibility of considering this mode of transmission in IM.