Diffinnir clefyd prin fel cyflwr, sy’n effeithio ar lai na 2,000 o bobl. Ar hyn o bryd, amcangyfrifir bod mwy na 7,000 o glefydau prin, gyda chyflyrau newydd yn cael eu nodi wrth i ymchwil barhau. Er bod clefydau prin yn brin yn unigol, ar y cyd maen nhw’n gyffredin, gydag un o bob 17 o bobl yn cael eu heffeithio gan glefyd prin rywbryd yn eu bywydau.
A rare disease is defined as a condition, which affects fewer than one in 2,000 people. It is currently estimated there are more than 7,000 rare diseases, with new conditions being identified as research continues. Although rare diseases are individually rare, they are collectively common, with one in 17 people being affected by a rare disease at some point in their lives.