As you might imagine, there are no diseases that affect a class of people simply because they're members of that class.
Simplifying a lot, disease is environmental or genetic. An example of an environmental disease that would strike a "class" of people would be Black Lung Disease, affecting coal workers, often numbered among the working poor class. (But people don't get it because they're among the working poor, but because they work in the mines....)
I believe what you want will be genetic, but before I go on, let's make something clear. I'm not talking about a genetic disorder. That would be something like autism. That's not a disease. It's not contagious. And you need to think about that, because all diseases are, by definition, contagious.
I therefore assume you want something that's contagious (acting like a real disease) but that's unlikely to be found in the non-noble classes. Not that it isn't impossible, just that it's improbable for some reason that relates to nobility. Ah! That means we're back to environmental. There's something about being noble that makes one more susceptible to this terrifying disease! Let's call it "The Dreaded Gombuu." But before we head over to The Dreaded Gombuu, let's close that discussion about genetics.
You need something that raises the odds of a noble dying from the disease considerably. So much so that people notice! They may even call The Dreaded Gombuu "the nobleman's curse!" In short, you need a disease with a genetic predisposition to the disease. (See also this.)
And you don't necessarily want to explain it. In fact, you don't want to explain it.
I strongly recommend that you not get caught in the trap of explaining the disease or the genetic predisposition. Oh, you might choose a particular disease (e.g., Meningitis) and you may choose how the genetic predisposition came to be (e.g., inbreeding), but if you think about it, people in a roughly medieval world wouldn't understand either one of those things in the first place. What they would be reacting to are symptoms and patterns. And that's what you need to focus on. That's what would make your story cool: the clues are all there, but not the answers.
So, if you want a disease that kills someone quickly, pick one! The only thing that matters is that the symptoms and progression of the disease are good for your story. The top-7 contenders for The Dreaded Gombuu from that link are:
- Flesh eating bug
- Stroke (not contagious, but has enough environmental factors that the requirement can be ignored — hallelujah for red meat and sugar! Both of which nobles would have access to more than peasants.)
- Pneumonic Plague
- Dengue haemorrhagic fever
But to be honest, it could be the common cold given the medieval era. A great many diseases we believe are irritating inconveniences were deadly back then due to bad hygiene, nutrition, and low understanding of basic medical practices.
After that, pick how the genetic predisposition to that disease comes to be. The easy one is inbreeding, but it could also be a "genetic defect" from a particular ancestral bloodline (curse the Abrovonavich family from Transylvania!) or truly environmental as it may be a consequence of babies spending tons of time in the presence of something that only the noble would decorate with. I don't know, like asbestos but not as well known. All you need is something that can believably rationalize the genetic predisposition that can be casually mentioned in the course of a story. ("Margaret was well-known for the distinctive Abrovanavich eyebrows....")
And that gives your readers the clues. Clues are good.