There’s no common worldwide definition of what constitutes a rare disease. The European Commission on Public Health defines rare diseases as “life-threatening or chronically debilitating diseases which are of such low prevalence that special combined efforts are needed to address them.” In Japan, the legal definition of a rare disease is one that affects fewer than 50,000 patients. (About 1 in 2,500). In the USA, the Rare Diseases Act also defines rare diseases according to prevalence, specifically “any disease or condition that affects fewer than 200,000.” (About 1 in 1,500).