The Disease Ontology  contains data associating genes with human diseases, using established disease codes and terminologies.
Statistics can now be found in the footers of each table in the GREAT web output.
All ontology data was downloaded and built between Oct 14, 2011 and Feb 18, 2012.
GREAT v1.8 Disease Ontology statistics are available here.
Osborne, J.D. et al. Annotating the human genome with Disease Ontology, BMC Genomics 2009 10:S1-S6