Before revision: We present a formulation of the SEIR concept that represents the evolution of an epidemic under the assumption of homogeneity and uniformity of the microscopic parameters: rate of infection, incubation and recovery/removal times. Non-homogeneities in the basic parameters are easily included by integration for arbitrary distributions  of incubation and removal times , recovering the classical SEIR model under the assumption of exponential distributions .
After revision: The paradigm for compartment models in epidemiology assumes exponentially distributed incubation and removal times, which is not realistic in actual populations. Commonly used variations with multiple exponentially distributed variables are more flexible, yet do not allow for arbitrary distributions of incubation and removal times , recovering the classical SEIR model under the assumption of exponential distributions .