Figure 1. . Seasonal variation in the occurrence of three human pathogens in the United States. A: an annual cycle of rubella activity was maintained between larger epidemics, which occurred every 6 to 9 years. B: the percentage of specimens testing positive for influenza viruses among specimens tested by World Health Organization and U.S. National Respiratory and Enteric Virus Surveillance System collaborating laboratories. C: a consistent pattern of rotavirus seasonality is evident in the U.S. National Respiratory and Enteric Virus Surveillance System. Adapted from references 4-6.