Chronic conditions illustrate the complex relationships that contribute to the risks of ill health and disease, and shape their outcomes. As we have already illustrated, social organization, economy and politics, as well as health systems factors, interact with pathogens and biology to determine the epidemiology and chronicity of disease. These factors influence episodes of comparative illness and stable health, the progress of disease, decline, and functional impairment. Many people with chronic conditions also have other unrelated ongoing health conditions, frequently associated with the same personal and socioeconomic preconditions. At the same time, many co-occurring conditions interact biologically and in relation to structural factors. As we noted in Chapter 1 , Merrill Singer (2009) developed the concept of syndemics to capture these multiple interactions, where two or more diseases interact at the level of biology, but also, concurrently, the diseases and structural factors interact, with the synergy resulting in an especially high prevalence and/or different mutations and expressions of the diseases (see also Ostrach and Singer 2012; Singer 2014; Singer and Bulled 2014). Such syndemics arise especially in extreme settings, as occur in periods of war, in fragile states, or in societies with marked inequalities. But outside of these circumstances too, people may experience a number of chronic conditions, often co-occurring and interrelated, and they struggle to manage these and head off complications. The result, for many, is a downward economic spiral and growing social exclusion, as accrued poor health inhibits workforce participation; as care, treatments and medications eat up available financial resources; and as poverty and illness combined erode family resilience.