Over the last 10 years, the field of mathematical epidemiology has piqued the interest of complex-systems researchers, resulting in a tremendous Periodicals, Inc. Complexity 14: 12-25, 2009 Key Words: epidemic; network; basic reproductive ratio; local asymptotic stability; spectral radius approximation; spectral graph theory; uncertain network
THRESHOLDS IN DISEASE MODELSA major objective of mathematical epidemiology is to serve public health interests by modeling the essential characteristics of disease transmission. Until the acceptance of the germ theory of infection in the late nineteenth century, disease modeling was limited to a posteriori statistical analysis of outbreaks. Once the biological mechanisms of infection propagation were understood, dynamic modeling became the approach of choice. The central modeling paradigm of mathematical epidemiology is the compartmental model, which tracks individuals as they transition between different disease compartments, classified by the compartment's ability to acquire and transmit infection. The general model will be described in Section 2, but an orienting example is shown in Figure 1, which depicts a simple model for a susceptibleinfected-susceptible (SIS) infection transmitted between two subpopulations. The Researchers are often interested in using disease models to determine whether or not a disease will become an "epidemic." In public health, this term is often loosely defined as "any marked upward fluctuation in disease incidence or prevalence" [1]. Often, researchers refer to a disease progression as an epidemic if a small initial infective population can grow in size, whereas others associate an epidemic with the establishment of an endemic presence, i.e., a sustained positive level of infection.
This article was submitted as an invited paper resulting from the "UnderstandingIn stochastic models, one is often interested in the time scales over which the disease is likely to be present. For example, Ganesh et al. identify sufficient conditions for the expected time to extinction of an SIS infection to be of order log(n) (fast die-out, no epidemic) on a network of n nodes, or of order exp(n α ), α > 0 (slow die-out, or effectively endemic) [2]. For additional interpretations of "epidemic" in stochastic models, see the work of Nasell [3], Newman [4], and Miller [5].Although the spread of infection is ideally modeled stochastically, as an individual-to-individual phenomenon, stochastic models can quickly become analytically intractable. Indeed, many results for these models are derived in the large-population limit, at Local stability of disease-free equilibrium [13][14][15][16][17][18][19][20] Existence of an endemic equilibrium [21][22][23] Multiple criteria [24][25][26][27] which point the stochastic behavior is well-approximated by a corresponding deterministic model (for a more precise statement, see the work of Kurtz [6,7] and Jacquez and Simon [8]). For the remainder of this article, we will explicitly focus on deterministic models, but the results ...