A conceptual model of the relationship between host susceptibility and pathogen virulence, first proposed by Scrimshaw and colleagues . The horizontal line represents the natural range in host susceptibility while the vertical line represents the natural range in pathogen virulence. As both are genetically modulated, nutrition is likely to have little or no effect at the extremes: when a pathogen is virulent or avirulent, or when the host is highly susceptible or insusceptible. Nutrition is most likely to have an effect when virulence and susceptibility are intermediate so that other factors may act. This is represented as the shaded area in the centre because the probability that nutrition has an effect on both characteristics is greatest in the middle of both ranges.