Fig. 2: Framework and hypotheses for the relationship between per parasite pathogenicity (PPP), exploitation, virulence and clearance rate.

a–e Host efforts to clear the pathogen generate costs and benefits via changes in a clearance rate and c immunity costs. b, d Clearance rate and immunity costs affect host survival and, e, in combination lead to an optimal host clearance effort (vertical dotted line) that maximises host survival. Host clearance effort is conceptualised as the strength of the resistance mechanisms that act towards pathogen clearance. f Schematic for how PPP, exploitation, and host clearance effort affect virulence and clearance rate: combined, these determine the three infection outcomes. This scheme extends the framework proposed by Råberg & Stjernman³¹, which proposes that PPP and exploitation are different determinants of virulence. The green and yellow labelled arrows represent our hypotheses and predictions. g–j Illustration of hypotheses H1 and H2, which are based on the idea that PPP and exploitation can each increase virulence. Two scenarios are depicted: low PPP or exploitation, and high PPP or exploitation (see legend). g The relationship between host clearance effort and clearance rate is not affected by high/low PPP or exploitation. h However, increased PPP or exploitation lower host survival. i These differential survival benefits of host clearance effort lead to the hypotheses that increases in PPP (H1) and exploitation (H2) lead to an increase in the optimal host clearance effort. j As a result, increased clearance rates are predicted with increasing PPP and exploitation (predictions P1 and P2 respectively). k–n Illustration of hypothesis H3, which is based on the idea that in addition to affecting virulence (g–j), higher exploitation makes it harder for the host to clear the infection. k Thus, for a given host clearance effort the clearance rate should be lower for higher exploitation, which results in lower host survival (l). m When considering total host survival, these differential survival benefits do not necessarily affect the optimal host clearance effort. n However, due to its direct influence on clearance rate, higher exploitation is predicted to lead to a lower clearance rate (P3). See Supplementary Note.