  We propose a novel SIR epidemic model which is driven by the transmission of
infection packets in networks. Specifically, infected nodes generate and
deliver infection packets causing the spread of the epidemic, while recovered
nodes block the delivery of infection packets, and this inhibits the epidemic
spreading. The efficient routing protocol governed by a control parameter
$\alpha$ is used in the packet transmission. We obtain the maximum
instantaneous population of infected nodes, the maximum population of ever
infected nodes, as well as the corresponding optimal $\alpha$ through
simulation. We find that generally more balanced load distribution leads to
more intense and wide spread of an epidemic in networks. Increasing either
average node degree or homogeneity of degree distribution will facilitate
epidemic spreading. When packet generation rate $\rho$ is small, increasing
$\rho$ favors epidemic spreading. However, when $\rho$ is large enough, traffic
congestion appears which inhibits epidemic spreading.
