orphan disease
n.— «The WHO has termed genetic and orphan diseases (an orphan disease is defined as a condition that affects less than 200,000 people nation-wide) as a major future health challenge.» —“Stress on diagnostic shift” Gulf Times (Doha, Qatar) Apr. 9, 2005. (source: Double-Tongued Dictionary)
Further reading
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A young listener once asked if there was a single word for “a combination of being nervous but also excited” about something. Listeners offered several of their own coinages, including nervouscited. Another helpful term along these lines...