罕见病，或称罕见疾病，是指仅在极少数人身上发生的稀罕病症，所以也被称为孤儿病。

大部分的罕见病都是遗传病，即使疾病症状不会生来就有，也会伴随患者终身。许多罕见病在患者生命早期发病，大约有30%患有罕见病的儿童会在5岁之前死亡。广为人知的罕见疾病如苯酮尿症、重型地中海贫血、成骨不全症、黏多糖症、高血氨症、有机酸血症、威尔森氏症等。核酸5磷酸异构酶缺乏病被认为是最为罕见的遗传病，仅发现一例患者。

患病人数达什幺水平才可称为罕见病目前并无一致意见。罕见病可以是地区性的，或者在该地区的某些特殊人群中是罕见病，而在另一些地区或人群，可能就是常见病。

A rare disease is any disease that affects a small percentage of the population.

Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not immediately appear. Many rare diseases appear early in life, and about 30 percent of children with rare diseases will die before reaching their fifth birthday. With a single diagnosed patient only, ribose-5-phosphate isomerase deficiency is considered the rarest genetic disease.