Table 1.

Definition of model parameters

parameter	definition
${\mathrm{z}}_F,{\mathrm{z}}_I$	Expressed individual trait values of immunity (fibrosis) and parasite infection status
$a_F,a_I$	Individual breeding values of fibrosis response (in absence of infection) and parasite infection; the latter reflects the encounter rate with parasites
${\mathrm{a}}_w$	fitness intercept
${\beta }_F$	Fitness cost of expressing immune trait ${\mathrm{z}}_F$
${\beta }_I$	Direct fitness cost of parasite infection ${\mathrm{z}}_I$
$\gamma$	Fitness benefit of immunity
${\phi }_{F,I}$	Sensitivity of expressed immunity to parasite infection; the degree to which expressed fibrosis depends on upon parasite infection
${\phi }_{I,F}$	Sensitivity of parasite infection to expressed immunity; the degree to which parasite infection depends on upon fibrosis
${\theta }_{\phi }$	Optimum immune sensitivity${\phi }_{F,I}$