Seasonal cycles of infectious diseases are universal and no single theory based on pathogen appearance, disappearance, or environmental changes has proved satisfactory. In June Emerging Infectious Diseases Scott Dowell of the Centers for Disease Control and Prevention in Atlanta suggests a novel theory, saying that the seasonal variation in infectious disease outbreaks are related to changes in host susceptibility mediated by the annual light/dark cycle and patterns of melatonin secretion.

Dr Dowell analysed the patterns of seasonal occurrence of common infections and found that changes in the prevalence or virulence of the pathogen cannot explain the simultaneous appearance of outbreaks across widespread geographic regions. In addition, the pathogens can be detected in the off-season without epidemic spread. Instead, Dr Dowell suggests that the annual light/dark cycle may induce seasonal changes in the expression of receptors on the surface of epithelial cells, as well as changes in lymphocyte profiles. These changes...

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