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Volume 7, Number 3—June 2001

Perspective

Seasonal Variation in Host Susceptibility and Cycles of Certain Infectious Diseases

Scott F. DowellComments to Author 
Author affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA

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Observations on the seasonal occurrence of infectious diseases

Observation Examples
Pathogens peak at characteristic times in all seasons of the year Winter: influenza, pneumococcus, rotavirus
Spring: RSV, measles
Summer: polio, other enteroviruses
Fall: parainfluenza virus type 1
Timing and duration of peaks for each pathogen are similar from year to year Measles: regular pattern since 1703 (1)
Influenza: annual peak varies by only 5 to 10 weeks in the United States (6)
Onset of epidemics often occurs simultaneously in areas that are geographically dispersed and have different weather conditions and diverse populations Influenza: simultaneous outbreaks across North America, 16 European countries, and 6 Chinese provinces (7)
Pneumococcus: simultaneous outbreaks in seven surveillance areas (8)
Latitude is a critical determinant of timing and magnitude of peaks An increasing magnitude of seasonal peaks as distance from the equator increases has been documented for polio (9) and rotavirus (10) and reported for influenza (11).
Pathogens can be detected in the off-season despite lower incidence of disease and virtual absence of epidemics Meningococcus: no decrease in carriage in the off-season, despite absence of epidemic disease (12)
RSV: sporadic summer viral isolation but no epidemic spread (13)
Influenza: sporadic summer isolation, occasional clusters of disease without epidemic spread (14)

RSV = respiratory syncytial virus. RSV peaks in the winter or spring in the United States, depending on location. For simplicity, it is listed here as a spring pathogen.

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