The idea that the rate of production of new infections, of directly transmi
tted microparasites, is a reflection of an interaction between the number o
f infectious hosts and the number of susceptible hosts in a population is c
entral to our understanding of dynamical processes in epidemiology. Empiric
al support for the idea has been seriously lacking, especially for endemic
infections in natural hosts, but is provided here for cowpox virus infectio
n in bank voles (Clethrionomys glareolus), using alternative descriptions o
f transmission dynamics: true and pseudo mass action. The former appears, o
verall, to give a better description of the dynamics. Transmission rates di
ffer in two natural populations, but both support previously untested theor
ies on the allometric scaling of transmission rates.