The calendar of epidemics: Seasonal cycles of infectious diseases.

Abstract

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 ​Tables11–4. Each acute infectious disease has its own seasonal window of occurrence, which, importantly, may vary among geographic locations and differ from other diseases within same location. Here we explore concept of an epidemic calendar, which is idea that seasonality is a unifying feature of epidemic-prone diseases and, in absence of control measures, local calendar can be marked by epidemics (Fig 1). A well-known example of a calendar marked by epidemics is that of Northern Hemisphere, where influenza outbreaks occur each winter [2, 3] (hence colloquial reference to winter as the flu season). In contrast, chickenpox outbreaks peak each spring [4, 5], and polio transmission historically occurred each summer [6]. Open in a separate window Fig 1 Epidemic calendar. The concept of an epidemic calendar is illustrated in top panel. Infectious diseases are seasonal, especially occurrence of acute and epidemic-prone diseases. In any given population, infectious diseases are distributed throughout year. Annual cycles of infectious disease are a ubiquitous feature of infection (Tables ​(Tables11–4). The illustration depicts wintertime seasonality of flu, springtime peaks of varicella (i.e., chickenpox), and summertime occurrence of gonorrhea and polio, in Northern Hemisphere. The bottom panel is a SIR schematic for transmission of human infectious diseases, which include acute and chronic diseases, those that are vector-borne, and those that are zoonotic with animal reservoirs. The vector, human, and reservoir hosts populations are partitioned into individuals who are susceptible to infection, infected, and recovered and immune. Seasonality may enter into any of eight key elements of system: (1) susceptible recruitment via reproduction, (2) transmission, (3) acquired immunity and recovery, (4) waning immunity, (5) natural mortality, (6) symptomatology and pathology (which may be acute or chronic, depending on disease), (7) disease-induced mortality, and (8) cross-species transmission. Disease illustrations reproduced from Google Medical Information. I, infected; R, recovered and immune; S, susceptible.