Figure 1. Seasonal movement and infectious disease dynamics.

A) Schematic incidence of an infection (blue line) showing cases (y axis) against time of year (x axis). A seasonal peak of incidence follows the summer. The underlying driver of this seasonal cycle is changes in the net reproductive number R_E, or the number of new infections per infectious individual, because R_E is the product of the basic reproductive number (R₀, or the number of new infections in a completely susceptible population), and S, the size of the accessible susceptible population, which varies seasonally following human behavior, but also susceptible depletion by infection. Two possible scenarios are illustrated: B) Aggregation of children in schools after the summer holiday (red arrows) increases effective contact among susceptible individuals, thus altering R_E and increasing incidence (reported for many childhood infections [1, 50, 54]); or C) Aggregation in cities after the period of agricultural work where communities have migrated out to rural settings (red arrows) results in an increase in R_E (reported for measles in Niger [5])