Seasonal cycles of infectious diseases are universal and no single theory based on pathogen appearance, disappearance, or environmental changes has proved satisfactory. In June
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 ...