ABSTRACT
Communitywide outbreaks during which the annualized incidence is markedly elevated over a period of several months have been recorded in Japan (20), Korea (21), and North America (22). In Japan, at least one national pandemic has occurred, but in North America there has been little evidence of spread from one geographic area to another. Outbreaks tend to be communitywide with no evidence of point source exposure or association with any particular group, school, or event. Epidemics have tended to occur in the winter and spring months, although there is no strong seasonal distribution of endemic cases in either temperate or subtropical zones.
