Figure 7: Weak environmental forcing. Seasonal forcing (18) combined with loss of individual immunity (Figure 4; Figure 1, 𝑘 r e c = 1  day−1). The system is initially in a steady state without forcing (Figure 4(b)); forcing is applied after 1 year. The effects of seasonal forcing on the basic reproductive rate 𝑅 0 (20) are shown in parts (a)–(c) (right-hand 𝑦 -axis). Increases in mean annual air temperature 𝑇 a i r , m e a n increase the level of forcing ((18), (19), Figure 6(a)). 𝑅 0 is the mean annual value of 𝑅 0 . Parts (c) and (d) are from a single simulation and have the same level of forcing.