I haven't been able to find a good source, but here's what the NCBI says: "In part because data on many conditions are limited to case reports or special popula

I haven't been able to find a good source, but here's what the NCBI says: "In part because data on many conditions are limited to case reports or special population studies, no well-supported estimate exists for the number of people collectively affected by rare diseases. A 1989 government report stated that 10 million to 20 million Americans had a rare condition (NCOD, 1989); the corresponding estimates in 2009 range from 25 million to 30 million (see, e.g., ORDR, 2009). The estimates were not accompanied by analyses or substantive citation of sources." http://www.ncbi.nlm.nih.gov/books/NBK56184/ http://www.ncbi.nlm.nih.gov/books/NBK56184/ The links in the footnotes lead to 404s, but this might be what they're referring to: http://rarediseases.info.nih.gov/asp/resources/ord_brochure.html http://rarediseases.info.nih.gov/asp/resources/ord_brochure.... It doesn't cite sources though and it calls it an estimate. It also says disease or condition, so I'm guessing it includes minor stuff that most people wouldn't really consider a disease. For example, my friend and her mom both have weird ridges on their thumbnails; definitely rare but it doesn't affect their health. Things like that are probably included in the estimate.