Economic Crises and Infectious Diseases Figure 1. Susceptible, Infectious, Recovered (SIR) Model of Infectious Disease Dynamics. Notes: Selected examples of determinants or drivers of ID transmission risk have been added to the SIR Model; however, they do not represent an exhaustive inventory and are intended to visualize potential impacts of the crises. Arrows represent suggested direction of these impacts. Extensions to the basic model for infectious such as TB can account for how factors that will increase progression are different from those that increase infection (as in the SIR model or susceptible, latent, infected TB models); see for example Blower S, McLean, AR, Porco, TC, et al. The intrinsic transmission dynamics of tuberculosis epidemics. Nature Medicine. 1995;1:815–21. doi:10.1371/journal.pone.0020724.g001 are susceptible to the disease, those who are infected and those who have recovered and are immune. The transition from ‘S’ (susceptible), to ‘I’ (infected) depends on the transmissibility of the disease and the rate of contact between susceptible and infected persons where ‘I’ is the infectious period, including its determi- nant, the virulence of the pathogen, which determines the rate of death. Finally, ‘R’ (removed), is the rate of recovery (or death), which is to some degree dependent on the availability of treatments which modify the duration of infectiousness. The rate of transition between these states, S to I to R, determines the course of an infectious disease epidemic. This framework can easily be applied to most common childhood diseases but might not be applicable to others (such as gonorrhoea); thus the SIR model is used here solely to frame the discussion rather than to describe the natural history of disease. Although the SIR model may lack detail for specific diseases (such as durations of latency, or partial immunity, etc.), it does enable an understanding of how an economic crisis could lead to rises (as well as potential reductions) in infectious disease transmission risk. It allows one to break down the complexities of economic crises to pinpoint key components that may be amenable to further study. For example, the SIR model helps to account for some of the dynamics of infectious disease spread in Eastern Europe during the transformational depression of the 1990s: Increasing effective contact rates (S to I). tick-borne encephalitis rose because it appears that people in the Baltic States spent more time in rural areas, coming into greater contact with ticks [13]; rises in incarceration placed high-risk groups into high- risk transmission conditions for TB, driving an epidemic in Eastern Europe [33,34]. Greater sub-group susceptibility (S to I). stress related to economic crisis increased high risk behaviour, consumption of alcohol, tobacco, substance abuse, and worse nutrition, all risk factors or the spread of some Ids [35,36,37,38,39,40]. Reductions in treatment access and quality (I to R). drops in government spending shift more costs onto patients and lead to disruptions to treatment and reduced quality of care; migration can lead to losses to follow-up [41,42]. PLoS ONE | www.plosone.org 3 June 2011 | Volume 6 | Issue 6 | e20724