Fig 1. Epidemic calendar.

The concept of an epidemic calendar is illustrated in the top panel. Infectious diseases are seasonal, especially the occurrence of acute and epidemic-prone diseases. In any given population, infectious diseases are distributed throughout the year. Annual cycles of infectious disease are a ubiquitous feature of infection (Tables 1–4). The illustration depicts the wintertime seasonality of flu, springtime peaks of varicella (i.e., chickenpox), and the summertime occurrence of gonorrhea and polio, in the Northern Hemisphere. The bottom panel is a SIR schematic for the 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 the eight key elements of the 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 the 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.