  We show that the mean field equations for the SIR epidemic can be exactly
solved for a network with arbitrary degree distribution. Our exact solution
consists of reducing the dynamics to a lone first order differential equation,
which has a solution in terms of an integral over functions dependent on the
degree distribution of the network, and reconstructing all mean field functions
of interest from this integral. Irreversibility of the SIR epidemic is crucial
for the solution. We also find exact solutions to the sexually transmitted
disease SI epidemic on bipartite graphs, to a simplified rumor spreading model,
and to a new model for recommendation spreading, via similar techniques.
Numerical simulations of these processes on scale free networks demonstrate the
qualitative validity of mean field theory in most regimes.
