Editorial: Direct and inverse comorbidities between complex disorders

Comorbidity and multimorbidity, defined as the presence of more than one disease in individuals, have emerged as a major challenge in the last decade (Valderas et al., 2009). Indeed, researchers, health professionals, healthcare managers and policy makers, and patients and citizens are lagging behind considering the comorbidity scenario, as illustrated by the paucity of documentation concerning interventions in people with multiple conditions (Smith et al., 2012). There is a clear need to better understand disease-disease relationships, in order to better organize and provide care, but also to develop appropriate research models. We can first characterize direct multimorbidity (higher-than-expected co-occurrence of diseases) and inverse multimorbidity (lower-than-expected co-occurrence of diseases). Examples of such disease associations include for instance diabetes mellitus associated with increased co-occurrence of depression (Rotella and Mannucci, 2013), or Alzheimer's disease associated with decreased co-occurrence of cancers (Catalá-López et al., 2014)