  General laws in ecological parasitology are scarce. Here we evaluate data
published by over 100 authors to determine whether the number of hosts in a
life cycle is associated with the degree of aggregation of fish parasites at
different stages. Parasite species were grouped taxonomically to produce 20 or
more data points per group as far as possible. Most parasites that remained at
one trophic level were less aggregated than those that had passed up a food
chain. We use a stochastic model to show that high parasite overdispersion in
predators can be solely the result of the accumulation of parasites in their
prey. The model is further developed to show that a change in the predators
feeding behaviour with age may further increase parasite aggregation.
