Introduction

Seasonal cyclicity is a ubiquitous feature of acute infectious diseases [1] and may be a ubiquitous feature of human infectious diseases in general, as illustrated in Tables 14. Each acute infectious disease has its own seasonal window of occurrence, which, importantly, may vary among geographic locations and differ from other diseases within the same location. Here we explore the concept of an epidemic calendar, which is the idea that seasonality is a unifying feature of epidemic-prone diseases and, in the absence of control measures, the local calendar can be marked by epidemics (Fig 1). A well-known example of a calendar marked by epidemics is that of the Northern Hemisphere, where influenza outbreaks occur each winter [23] (hence the colloquial reference to winter as “the flu season”). In contrast, chickenpox outbreaks peak each spring [45], and polio transmission historically occurred each summer [6].

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Citation: Martinez ME (2018) The calendar of epidemics: Seasonal cycles of infectious diseases. PLoS Pathog 14(11): e1007327. https://doi.org/10.1371/journal.ppat.1007327