Crystalisis

Crystalisis is a strange infection which causes crystallisation of the host. It is infectious and can infect most kinds of animals, although plants and fungi seem to be immune.

Crystalisis has a low chance of transfer through skin contact, but travels very effectively through contact with blood and body fluids. Only the crystal shards themselves are infectious.

Crystalisis starts crystallisation of the hosts blood (which can lead to heart attacks) and then spreads outward. It can be detected by magic before symptoms become apparent, but most people don't know they are infected until the crystal growths break through their skin.

Depending on where the infection starts, the results vary greatly, crystallisation of the head and brain areas lead to sight and memory problems, while gut growths effect digestion.

Crystal Types
Although they are all part of the same disease, the crystals that grow from infected people can develop in different colours, depending on the host.

Red
The most common form of crystalisis is the red form, it develops the most slowly out of all the different kinds

Blue
Blue crystalisis is an uncommon variety which exclusively grows from hosts with latent magic in their blood. With proper practice the infected can cast magic through their crystals, but this aggravates the infection and causes it to spread faster. A playable sorcerer subclass based on Blue Crystalisis is available here.

Treatment
Treatment of crystalisis is difficult, it is resistant to magic and cannot be fully cured using spells such as lesser restoration, though they do stop it's advancement temporarily. The most effective way of treating crystalisis found so far is amputation of the effected limb. Infections in the head or torso of a host are harder to deal with.

Exploitation
Despite the incredible danger crystalisis presents, as well as the stigma surrounding those infected by it, some sages have discovered uses for the raw crystals, this practice is outlawed almost universally.