Syndemics

Disease Interactions

Interest in a syndemics approach has been driven by growing evidence of significant interactions among comorbid diseases. One such interaction has been found, for example, between type 2 diabetes mellitus and various infections, such as hepatitis C viral infection. Several factors are known to contribute to the onset of type 2 diabetes, including diet, obesity, and aging. The role of infection, however, is only beginning to be understood. Already, it is known that risk for serious infections of various kinds increases significantly with poor diabetes control, but appreciation of more complex relationships between infection and type 2 diabetes is now emerging as well. The Third National Health and Nutritional Examination Survey (NHANES III) found that the frequency of type 2 diabetes increases among people who have been infected with the hepatitis C virus. Similarly, several health reports note that diabetes is present in as many as 37% of those who are critically ill with severe acute respiratory syndrome.

[Page 1025]The nature of interaction among diseases may vary and need not require direct physical interaction to produce new or amplified health consequences (e.g., as in AIDS, changes in biochemistry, or damage to organ systems caused by one pathogenic agent may facilitate the spread or impact of another agent). Direct interaction, however, including gene mixing among different types of pathogenic agents, has also been described, such as the molecular in vivo integration of the avian leukosis virus and Markek's disease virus (MDV) in domestic fowl. Both these cancercausing viruses are known to infect the same poultry flock, the same chicken, and even the same anatomic cell. In coinfected cells, the retroviral DNA of the avian leukosis virus can integrate into the MDV genome, producing altered biological properties compared with the parental MDV. In studies of human populations, a lethal synergism has been identified between influenza virus and pneumococcus, a likely cause of excess mortality from secondary bacterial pneumonia during influenza epidemics. It is disease interactions of this sort that are a central biological component in syndemics. Syndemic theory seeks to draw attention to and provide a framework for the analysis of these interactions.

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