''You think because you understand 'one' you must also understand 'two', because one and one makes two. But you must also understand 'and'.'' At the end of this lecture you will understand the core features of a syndemic process and how a syndemic approach goes beyond the comorbidity approach. The word syndemic was created in the mid 1990s by the founder of the syndemic approach, Merrill Singer. He combined Greek words, in this case the prefix 'syn', from the Greek word synergos (s??e????) which means working together, and the suffix 'demic' from epidemic. Epidemic itself derives from the Greek words epi (?p?�), which means upon, and demos (d?�??) which means people. In his handbook on syndemics, Singer explains how he wanted the word to signify a neglected phenomenon in public health: The dynamic relationship involving two or more epidemics of diseases or health conditions, their working together, and the socio-environmental context that promotes their interaction. Three criteria set the boundaries that define when something is, or is not a syndemic. The first criterion is that there is clustering of two or more diseases or health conditions. Second, there is a synergistic adverse interaction between the clustering problems, leading to an increase health burden of the affected population. And third, contextual and social factors create the conditions for clustering and synergistic interaction. I will discuss these three criteria more in depth. First, clustering of two or more diseases or health conditions. What you see here is a population in which some people have a disease. In medicine, diseases are generally approached as distinct entities, in isolation from other diseases and the social contexts in which the disease occurs. And thus, physicians commonly search for one underlying health problem that explains all experienced symptoms the patient presents. In fact, most clinical guidelines address single diseases while all physicians know that some of their patients may have combinations of diseases. Here you see the people with more than one disease or health condition. If more than one disease is diagnosed in a patient, this is usually called comorbidity or multimorbidity: The patient suffers from different afflictions, that occur side-by-side. When certain combinations of diseases are prevalent in a particular population there is clustering of disease. Now you see that certain combinations occur more often in this population. Clustering of disease therefore refers to both the individual level, where health problems cooccur, and the population level where patterns of co-occurrence are identified. The first criterion of a syndemic is that the co-occurrence of two or more health problems has epidemic proportions in a specific population. Epidemic means that the rates of disease are reaching some predefined increase above the baseline level of the disease. Second, synergistic adverse interactions leading to an increase health burden. Clustering of disease in itself is not sufficient to warrant a syndemic perspective. The second criterion of a syndemic refers to the relationship between the clustering health problems: Diseases and health conditions are considered intertwined. This means that the natural course of one disease is altered due to the presence of another co-occurring disease or condition. This is what we call a disease-disease interaction. Only when two or more diseases or health conditions work together in an adverse manner, one hypothesize a syndemic. For instance, one disease may aid another in gaining access to a vulnerable area of the body. Or, one infectious disease may enhance the virulence of another infectious disease, or alter the body's biochemistry in such a way that it hampers the immune response to another disease. Similarly, mental illness and psychological distress may pave the way for other health problems or augment the health consequences. In a syndemic, the biological interaction of two or more diseases in individuals and within a population amplifies the disease burden: working together, they lead to more illness or disability, or a more rapid health decline in individuals than if there was no clustering of disease. At the population level, this amplification leads to a disproportionate increase of morbidity or mortality within the population. Third, the key role of contexts. In syndemic research the answer to how diseases become intertwined, does not only concern the biological processes in the bodies of patients, but also considers their social environment. The key role of the social context is the third criterion a syndemic has to meet. Clustering of and interactions between diseases or health conditions do not occur in isolation. The spread, clustering, and adverse interactions are shaped by encompassing social conditions such as structural violence, poverty, stress, and stigmatization. These may all amplify the vulnerability of disadvantaged individuals and populations by increasing susceptibility, reducing immunological functions, and limiting people's resources to prevent the negative spirals from happening. The pathways to epidemics of clustering diseases are complex. To identify a syndemic, first, we must establish epidemic patterns of comorbidities, the clusters of diseases or health conditions. Second, it must be confirmed that the adverse synergistic disease-disease interactions lead to increased health burdens in individuals and populations. And finally, the interaction of social conditions in the causal pathways to clustering and interaction must be demonstrated. It is now clear that a diagnosis of comorbidity alone stops short of a diagnosis of a syndemic. In comorbidity, diseases co-occur with other diseases as separate entities. But a syndemic is one biosocial complex in which diseases and health conditions interact with one another and with social conditions. In my lecture, 'themes in syndemic research', I will present some examples of such biosocial complexes.