Exhaustive list of rare and orphan diseases. Orphan diseases are rare diseases that are not widely present in the population. Rare diseases are cost prohibitive to treat as medicines and procedures are expensive to create and not enough people are affected to cover the cost of production.Some of these may not even have mature treatments to combat. For some more obscure diseases check out the enteritis symptoms, the sacroiliitis symptoms and the cerebral edema symptoms. So what are some rare genetic disorders? This list will serve as your reference guide for that answer.