  Recent work by Erik Volz [arXiv:0705.2092] has shown how to calculate the
growth and eventual decay of an SIR epidemic on a static random network,
assuming infection and recovery each happen at constant rates. This calculation
allows us to account for effects due to heterogeneity in degree that are
neglected in the standard mass-action SIR equations. In this note we offer an
alternate derivation which arrives at a simpler -- though equivalent -- system
of governing equations to that of Volz. This new derivation is more closely
connected to the underlying physical processes, and the resulting equations are
of comparable complexity to the mass-action SIR equations. We further show that
earlier derivations of the final size of epidemics on networks can be
reproduced using the same approach, thereby providing a common framework for
calculating both the dynamics and the final size of an epidemic spreading on a
random network.
