An important issue in the history of ecology has been the study of the
relative importance of deterministic forces and processes noise in shaping
the dynamics of ecological populations. We address this question by exploring
the temporal dynamics of two childhood infections - measles and whooping
cough - in England \& Wales. We demonstrate that epidemics of whooping
cough are strongly influenced by stochasticity; fully deterministic approaches
cannot achieve even a qualitative fit to the observed data. In contrast,
measles dynamics are extremely well explained by a deterministic model.
These differences are shown to be caused by their contrasting responses
to dynamical noise, due to different infectious periods.