From Omics.org

 The diseasome is network in which disorders are linked to the respective associated disease genes (Goh et al, 2007 PNAS). 
Two projections of the network are presented: a) the Human Disease Network (HDN), in which diseases are connected to each other if they share a common disease gene; b) the Disease Gene Network (DGN), in which genes are connected if they are associated with a common disease. The HDN has a giant component comprising almost half of the diseases, in which some classes of disorders cluster naturally (eg cancers or neurological disorders, but not metabolic disorders). The DGN, when integrated with functional annotations, expression and protein-protein interaction data, provides a first step towards a "network-based explanation of the emergence of complex polygenic disorders" in the sense that it reveals how functionally related genes can lead to similar disorders.