When a magical being or person gets hit with a disease that affects their powers and themselves physically as well, how is it handled?
I am strictly talking about the diseases and viruses that can harshly affect a magical person or being in a way that is different from a common cold, flu, cancer, or something else along those lines.
Thoughts and feelings?
NOTE: I have a feeling like I am thinking about myself solely…sorry.
YES
I do 
in my Oz-inspired world, Ozel Emla. It was the first world where I had the idea for a disease affecting magic.
The name of the disease is…er… checks notes
it’s a cringy name when I look at it now, but…
magius crystallizis
It’s a disease where magic crystallizes (oh wow, who could have guessed? 
sarcasm). When that happens, it can be removed at birth, or it can lodge into dangerous places like the heart. Taking it out would be fatal. The disease shortens lifespans, and using magic too much can easily exhaust the person. They can get headaches and nausea if they exert too much especially when they’re children.
The one upside to this is that they can do otherwise complex magic fairly easily dubbing many with this disease as a prodigy.
What even causes magic to crystallize like that?
Idk 
Haven’t thought that far.
I haven’t gotten into this character’s story. That’s probably where it will come up.
Oh…
LOL!
Yeah, that’s all I got
Anyone else want to try it out?
Oh geez how do I explain this one 
Okay, the only confirmed “disease” is a bit of a weird one. Honestly I’m not sure if it actually qualifies as a “disease”.
It affects those with Lyriumian / Terpolite blood. So, when a Lyriumian / Terpolite come into their power (usually in the 200s), they have to use their power once in a certain time period otherwise they get very sick from power buildup. If too much builds up, their power consumes them and they die.
This probably doesn’t sound like a big deal but there are Lyriumians / Terpolites who struggle to manifest their powers, usually a mental block or something nullifying them (I haven’t really explored everything that could interfere). Because of this, there are ways for these people to syphon their power back down to a “normal” state.
Of course there are normal colds and flus as well but those don’t normally have any impact on magical ability.
I am working on a new one that I’m yet to get the specifics of for the first book in The Photon Cycle. It’s very much still under construction
Only in in-depth stories.
Slightly off-topic question:
What would be an interesting twist for a magical illness that affects a magical being, personally?
Like what would a different sort of way a magical being could get afflicted with a magic-based illness, that only the most skilled could do?
I am so sorry if I am asking such weird ass questions. Is it too much? I was just curious to know your answer. LOL!
I was just looking at MLP infection AUs and now I see this?
What???
Yeah crazy! Have you been looking at my youtube watch history?
No.
So, magic in my world has been essentially corrupted. Long story short: the magical atmosphere was sufficiently damaged, which in turn corrupted magic and its users, which thus nearly wiped out the entire human race.
People refer to it as ‘The Void Plague’ it causes people who have magic to become infected with corruption. They grow physically and mentally sick (think something close to rabies), they first grow extremely tired, they cough up blood, they’re feverish and their blood burns–it’s an extremely painful disease. In the later stages, close to death, they begin to hallucinate, to become violent to people attempting to help them, they experience paranoia, all sorts of things. And then they die. There is no cure for it once you become infected with the Void Plague. In turn, it directly affects and is influenced by magic. The more they use magic, the more likely they are to become infected. If they use magic it becomes dreadfully unstable and unreliable to a dangerous degree.
Wooah that’s sick! Has anyone ever purposefully infected themselves in order to achieve that level of magic?
This kinda reminds me of The Lunar Chronicles by Marissa Meyer (highly recommend, its a fun series with a ScFi twist on princesses/fable stories–for context, Cinderella is a cyborg.) where a character experiences something akin to magical build-up and slowly goes insane because they refuse to use their magic and that’s not good.
Well, that’s terrifying…
Spooky spooky~~~