orphan disease
n.— «A 12-year-old Simi Valley boy with a rare immune system disorder has inspired a series of benefits to help find a cure for what is known as an “orphan disease” because it affects too few children to warrant federal research funds.» —“Simi boy’s rare disease inspires annual fundraising softball tournament” by Alicia Doyle Ventura County Star (California) June 23, 2008. (source: Double-Tongued Dictionary)
Further reading
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this term is related to the term “orphan drug”, which has been in common use for at least thirty years. Indeed it appears in my 25thEd (1990) Stedman’s, under orphan products: “drugs, biologicals, and medical devices… that may be useful in common or rare diseases, but which are not considered commercially viable.” The FDA has a special status for these drugs, which are usually used to treat, you guessed it, orphan diseases.