A closed and homogeneously mixing population is subdivided into four classes, the susceptibles (S), the latent infected (L), the infectious (I), and the immune or removed persons (R). It is assumed that infectious persons may have contacts with any other member of the population with equal probability. Any susceptible, once contacted by an infective, becomes latent infected, then infective and at last immune or removed from the infection process. In order to get a stochastic epidemic model
