# What's the rarest disease in the world? **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2003, 11:39pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595 "2003-12-02T23:39:13Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![NajaNivea](https://avatars.discourse-cdn.com/v4/letter/n/22d042/32.png) [@NajaNivea](https://boards.straightdope.com/u/NajaNivea)\ **Post date:** [December 3, 2003, 4:13pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595/21 "2003-12-03T16:13:25Z")
> [@](#): > > \*Originally posted by Chicago Faucet \* > **There is the disease of people who are allergic to sunlight. Yes, allergic. They break out in hives and have trouble breathing. It is strictly heriditary, and skips generations. There are two young daughters of an American couple that would have to take them outside to play between the hours of midnight and 4 am. Only about a hundred cases are known.** I lived in a very small town growing up, and these girls lived in the next small town over. Their car had extremely heavily tinted windows, and if the girls had to leave the house for any reason during the day, they’d have to cover every bit of skin surface, or they’d develop blisters and such, a really severe reaction. They would go to parks to play in the middle of the night. ---
**Author:** ![Nichol\_storm](https://avatars.discourse-cdn.com/v4/letter/n/3ec8ea/32.png) [@Nichol\_storm](https://boards.straightdope.com/u/Nichol_storm)\ **Post date:** [December 3, 2003, 6:25pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595/22 "2003-12-03T18:25:50Z")
> [@](#): > > \*Originally posted by Chicago Faucet \* > \*\*There is the disease of people who are allergic to sunlight. Yes, allergic. They break out in hives and have trouble breathing. It is strictly heriditary, and skips generations. There are two young daughters of an American couple that would have to take them outside to play between the hours of midnight and 4 am. Only about a hundred cases are known. > \*\* That’s xeroderma pigmentosum, IIRC. I once briefly met a young woman with the condition. > [@](#): > > _Originally posted by sugaree_ > **Her symptoms seem to be rather more common than we may wish. There are many people out there who never learn those skills due to mental retardation or brain damage. By physically an infant, do you mean that she not grown past the size of a infant?** Yes, that’s what I mean. She was physically identical to a child of about six months old. She simply… never grew up. Very, very strange. The parents had actually had two daughters after her birth who had aged normally and were then regular children while their _older_ sister remained trapped in babyhood. ---
**Author:** ![Bippy\_the\_Beardless](https://avatars.discourse-cdn.com/v4/letter/b/ac8455/32.png) [@Bippy\_the\_Beardless](https://boards.straightdope.com/u/Bippy_the_Beardless)\ **Post date:** [December 3, 2003, 7:08pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595/23 "2003-12-03T19:08:18Z")
Well lycanthropy has no known cases, but that is just because they havn’t caught me yet. \ ---
**Author:** ![PbZep](https://avatars.discourse-cdn.com/v4/letter/p/58f4c7/32.png) [@PbZep](https://boards.straightdope.com/u/PbZep)\ **Post date:** [December 3, 2003, 8:45pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595/24 "2003-12-03T20:45:49Z")
Wow, someone mentioned xeroderma pigmentosum. I did my grad work on a yeast gene whose human ortholog (when mutant) caused the disease. Anyway **Chastain86** took my answer, so I’ll also suggest chronic _harmonius matrimonium_. Back to being serius, I would bet, but have no data to back up, that there are _one off_ diseases that are close to unique, i.e. only one person has it or ever will. This starts to conjur up the question of, what exactly is the definition of a disease though… ---
**Author:** ![liirogue](https://avatars.discourse-cdn.com/v4/letter/l/dbc845/32.png) [@liirogue](https://boards.straightdope.com/u/liirogue)\ **Post date:** [December 3, 2003, 10:39pm UTC](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595/25 "2003-12-03T22:39:12Z")
> [@](#): > > \*Originally posted by nitroglycerine \* > \*\*Here’s a nice one: [Fatal Familial Insomnia]([http://Fatal](http://Fatal) Familial Insomnia). IIRC, there is an inherited variant that only exists in one bloodline from Italy. Google around…it’s a pretty horrible disease. > > Jon \*\* I can’t get your link to pull up. Anyone else having the same problem? [Previous page](https://boards.straightdope.com/t/whats-the-rarest-disease-in-the-world/216595.md?page=1)