Although S + E + I + R = 1 in the deterministic system, one should note that the dynamics of the stochastic SEIR system will not necessarily have all of the components sum to unity. However, since the noise has zero mean, the total population will remain close to unity on average. Therefore, we assume that the dynamics is sufficiently described by Eqs. ͑1a͒-͑1c͒. It should be noted that even if E͑t͒ + I͑t͒ = 0 for some t, the noise allows for the reemergence of the epidemic.