Figure 1.

Hypothetical associations in natural populations between prevalence of infection and infection intensity, respectively, and three different types of immuno-alleles: (a,b) a qualitative disease-resistance allele, (c,d) a susceptibility allele and (e,f) a quantitative disease-resistance allele. A qualitative disease-resistance allele is negatively associated with both (a) prevalence of infection and (b) infection intensity. A susceptibility allele is positively associated with both (c) prevalence of infection and (d) infection intensity. A quantitative disease-resistance allele is positively associated with (e) prevalence of infection but (f) negatively associated with infection intensity.