In this article, as a first study of a high-dimensional system, we consider the susceptible-exposed-infected-recovered ͑SEIR͒ epidemiological model with stochastic forcing. As previously mentioned, we could easily consider a SEIR-type model where the exposed class was modeled using hundreds of compartments. Since the analysis is similar, we consider the simpler standard SEIR model to demonstrate the power of our method. Section II provides a complete description of this model. Section III describes how to transform the deterministic SEIR system to a new system that satisfies the spectral requirements needed to apply the center manifold theory. After the theory is used to find the evolution equations that describe the dynamics on the center manifold, we show in Sec. IV how the reduced model that is found using the deterministic result incorrectly projects the noise onto the center manifold. Section V demonstrates the use of a stochastic normal form coordinate transform to correctly project the noise onto the stochastic center manifold. A discussion and the conclusions are contained, respectively, in Secs. VI and VII.