The idea that the rate of production of new infections, of directly transmitted microparasites, is a reflection of an interaction between the number of infectious hosts and the number of susceptible hosts in a population is central to our understanding of dynamical processes in epidemiology. Empirical support for the idea has been seriously lacking, especially for endemic infections in natural hosts, but is provided here for cowpox virus infection in bank voles (Clethrionomys glareolus), using alternative descriptions of transmission dynamics: true and pseudo mass action. The former appears, overall, to give a better description of the dynamics. Transmission rates differ in two natural populations, but both support previously untested theories on the allometric scaling of transmission rates.