There has traditionally existed a focus on whether transmission is frequency- or density-dependent, and rarely are other, nonlinear transmission–density functions investigated . Nevertheless, it has been well established that the binary distinction between density independence and linearity can be inadequate, and a range of other possible nonlinear transmission functions has been suggested, often in the power law, asymptotic or logistic family [10,22–25]. Cowpox virus dynamics in a natural population of field voles (Microtus agrestis), for example, has been shown to be best described by a nonlinear power function intermediate between density independence and linearity . Intermediate density-dependence was also observed in Ambystoma tigrinum virus transmission in the tiger salamander .