Abstract
During the last two decades, research on multimorbidity (e.g., the evaluation of the coexistence of multiple chronic diseases in the same person, whichever the diseases are) has rapidly increased. Previous reports have clearly shown that multimorbidity affects a large proportion of older persons ranging from 55 to 98% across studies depending on definition, age of the population and data source, making this condition to be defined as the ‘most common chronic disease’. A few possible risk factors for multimorbidity were identified, such as increasing age, whereas a large social network seemed to play a protective role. However, the exclusive use of a quantitative approach to this phenomenon fails to catch the nature and dynamics of the different patterns of co-existing diseases. Given that there are no clear boundaries between many diseases, studying the structure defined by the entire set of co-occurring morbidities might help to understand biological and medical questions.