outbreaks. During the financial crisis in Malaysia in 1996, the number of immunised children decreased substantially as a result of reduced contact with primary care services [61]. Declines in childhood diphtheria immunisations led to the re-emergence of diphtheria in the Former Soviet Union, the first large-scale epidemic in industrialised countries in the past three decades [70]. During an economic recession in the 1980s, the U.S. state of Colorado reported a 6% drop in the proportion of fully immunised two year olds, attributed to increased poverty rates and diminished health benefits and social support services [62]. Discussion Assessing the potential impacts of the recent economic turmoil on the spread and control of communicable disease is complicated by the absence of fully analogous historic economic circumstances, not to mention the absence of quantitative studies of disease spread during such downturns. Interpretation of the scientific literature is also complicated by the potential for publication bias. Yet the pathways through which economic havoc could have impacted on communicable diseases in the past, and therefore might do so again in the future, are important and numerous. Before proceeding to assess the evidence identified in the literature review and narrative synthesis, we must first note several important limitations of the study. First, nearly all studies typically lacked an underlying infectious disease framework (and often the necessary data) for analysing the mechanisms of risk and therefore the potential avenues for intervention. Among those studies reporting reductions in risk, plausible explanations have not been provided, perhaps resulting from this absence of a biological framework [57,64]. The available research may have missed some longer term consequences. This is inevitable given the myriad of other factors that may intervene over time. Second, there is likely to be some degree of publication bias, although the methodological issues discussed above prevent testing for this, for example by using a funnel plot. Such a bias could skew our findings to identify more negative effects than actually occurred, as findings or possibly even protective effects may not have been reported to the same extent as negative ones. Third, given the state of existing literature and heterogeneity in outcomes, exposure variables, and methods, we were unable to use quantitative meta-analytic methods to pool data from studies and infer effect sizes. To address these significant limitations of existing studies, we focused on identifying potential effects on infectious disease outcomes of economic downturns that may occur, categories of vulnerable groups, and, where possible assessing the extent to which outcomes may be dependent of identifiable elements of economic and social context. Acknowledging these important limitations of existing literature, our study provides a first effort to structure existing findings into a theoretical model of infectious disease spread, the SIR model. This framework may serve as a guide for future investigations on economic downturns and infectious disease outcomes. When viewing existing literature through the lens of the SIR model, we found that several studies identified an elevation of risks, both immediately (mainly in terms of increased direct contact rates) and with a delay of several years (in terms of indirect transmission through infrastructure deterioration or reduced health system capacity to provide effective treatment, resulting in longer infectious periods) [4,72]. These studies provide indications that economic downturns could exacerbate socio-economic inequalities while increasing certain susceptible populations and high-risk spreader groups, such as prisoners, migrants, and the homeless. Lower living standards during times of economic duress may also lead to over-crowding, poorer nutrition and, although the Economic Crises and Infectious Diseases evidence is inconclusive, potentially reduced immunity due to living with higher stress levels. Furthermore, environmental exposures to pathogens may be altered (e.g. subsistence farming or encroachment of wildlife into abandoned areas), creating new opportunities for the spread vector-borne disease. Recovery rates may be similarly affected, with less funding available for healthcare provision, and susceptible groups less able to access healthcare, particularly in health systems that do not offer universal coverage. These findings provide a cautionary note to policymakers and may facilitate infectious disease control planning in response to current and future economic crises. There is also evidence that the consequences of infectious diseases can also provoke negative economic effects, potentially impeding economic recovery. For example, high HIV burdens have been found to lead to economic instability and poor socio-economic growth prospects in several Sub-Saharan nations, while even relatively limited outbreaks, such as the 2003 SARS outbreak, are estimated to have cost the countries of East and South East Asia some 2% of GDP [73]. Models of the impact of influenza in the UK suggested costs ranging from 0.5% to 1.0% of GDP for low fatality scenarios and 3.3% and 4.3% for high fatality scenarios [74]. This significantly outweighed the economic costs of pre-pandemic vaccines, which were estimated to limit the overall economic impact. Taken together, these studies point to the possibility of a vicious cycle, in which poor health and poor economic growth perpetually undermine one another, either across whole societies or important sub-sets of society. From a policy perspective, it may be desirable to safeguard and perhaps even bolster infectious disease control budgets during economic downturns. Unfortunately, at a time in which public debt to GDP levels in advanced economies are as high as they have been since the 1950s, and at a time in which population aging is a pressing issue for many countries, the prospects for increasing public health financing are not strong. This may be particularly the case for recipient countries of international aid; Greece, for example, recently accepted European Union and IMF loans after agreeing to undertake rigorous reforms and austerity programmes, including to its health sector [75]. Reforms in and of themselves, of course, can be important opportunities to improve efficiency and service provision within public health systems [5]. However, as we noted earlier, adhering to strict austerity policies may risk a worsening of infectious disease outcomes [76]. Overall, this narrative synthesis highlights the need for better understanding and further quantification of the links between economic downturns and communicable disease spread. This will require research that is highly multi-disciplinary, combining insights from, among others, epidemiology, microbiology, vector biology, ecology, and social geography. Economic downturns may pose increased risks of infectious diseases for some groups in the population. Strategies to identify and engage high-risk groups, while also safeguarding already tight budgets, should be high priorities for the health sector, as communicable diseases, like economic crises, are difficult to control once they begin to spread [77]. Acknowledgments We would like to thank Ana-Belen Escriva for technical help with the literature search. Author Contributions Conceived and designed the experiments: JCS JES ST. Performed the experiments: M. Suhrcke DS MD M. Senek MM SB IA PH BR. Analyzed the data: M. Suhrcke DS JES MD M. Senek MM ST SB IA PH BR JCS. Wrote the manuscript: M. Suhrcke DS JES MD M. Senek MM ST SB IA PH BR JCS. PLoS ONE | www.plosone.org 10 June 2011 | Volume 6 | Issue 6 | e20724