We note that although MeSH category C is described as comprising diseases, many of the terms in the complete tree C (4,620 entries) do not refer to specific diseases. For instance, many of the terms describe general categories, such as “brain diseases” (MeSH: D001927), veterinary diseases (e.g., “brucellosis, bovine” [MeSH: D002007]), and various other entities, such as “cadaver” (MeSH: D002102). Others represent phenotypic features of diseases rather than actual disease entities; one example is “Cheyne-Stokes respiration” (MeSH: D002639), which is an abnormal breathing pattern that can be observed in diseases such as central sleep apnea syndrome. We excluded such MeSH entries by careful manual curation, leaving a total of 3,145 MeSH category C descriptors that we judged to actually represent specific disease entries. Only these entries were used for the analysis described in this manuscript.