  We show how the standard field theoretical language based on creation and
annihilation operators may be used for a straightforward derivation of closed
master equations describing the population dynamics of multivariate stochastic
epidemic models. In order to do that, we introduce an SIR-inspired stochastic
model for hepatitis C virus epidemic, from which we obtain the time evolution
of the mean number of susceptible, infected, recovered and chronically infected
individuals in a population whose total size is allowed to change.
