Table 1.

Predictions of mechanisms proposed to influence the shape of age–intensity (1–7) and age–distribution (4–10) relationships in host–parasite systems

Mechanism no.	Process	Mean intensity	Variance-to-mean ratio
1.	Seasonal or inter-annual variability in exposure	System-specific	No effectb
2.	Behavioral avoidance improves with age	Asymptotic (Type II)	No effect
3.	Innate changes in immunity with age	System-specific	No effect
4.	Acquired immunity following repeated exposure	Asymptotic (Type II)a	Reduced aggregation (<1), decreasing with agea
5.	Density-dependent parasite competition	Asymptotic (Type II)b	Reduced aggregation (<1), decreasing with ageb
6.	Density-dependent parasite facilitation	Exponential increaseb	Aggregated (>1), increasing with ageb
7.	Host mortality due to infection	Asymptotic (Type II) or convex (Type III)b,c	Reduced aggregation, decreasing with ageb,c
8.	Heterogeneity in exposure	No effectb,c	Aggregated (>1), increasing with ageb,c
9.	Heterogeneity in susceptibility	No effectb,c	Aggregated (>1), increasing with ageb,c
10.	Clumped infection	No effectb	Aggregated (>1), decreasing with ageb

^aAnderson and May (1985)

^bDuerr et al. (2003)

^cAnderson and Gordon (1982)