What interesting medical conditions exist in your world?

What are some interesting ailments you've invented? Probably with spoilers for the grosser details.

I have a couple. The first is my take on vampires. ::: spoiler Vampirism Vampirism in Ortharen is caused by a parasite called Esor crorum, commonly known as dracula (singular draculum). They spread through contact with blood and can infect most animals.

Dracula stop you from producing red blood cells but allows you to use blood you ingest, so if you don't drink the blood of unafflicted creatures you'll suffocate without a way to move oxygen around your body. Most blood will work for most vampires, but blood of your species and a compatible blood type works best.

This condition also stops you producing melanin and makes you more sensitive to bright lights, but that's less important than the blood thing.

Vampires are very resilient to other diseases because dracula will ruthlessly attack any other bugs you might catch. But not stuff you already had when you became a vampire, probably because it recognizes those as part of your body.

A living vampire can kill their dracula by eating lots of alliums with other immune boosting foods, but they still won't be able to produce new red blood cells and they'll need regular transfusions or they'll suffocate and die. :::

Then there's one about eyes.