The story takes
place in the future, after disease have been eliminated. People live in a
antiseptic world that strips the flavor and nuance out of life. The
story focuses on a performance artist who deliberately infects himself
with various horrific maladies and allows people to watch their symptoms
blossom. Despite the concerns of his medical assistant/lover, the
Disease Artist presses the boundaries of his performances in order to
reconnect his audience to their lost humanity.
As a genre, horror can be a collection of missed opportunities. On paper, it's supposed to be about tapping into hidden anxieties in order to create stories that press those buttons in a safe and entertaining way. So we create avatars for anxieties like the vampire for (depending on your politics) fears of plague or predatory sexuality, werewolves for fear of primal nature, ghosts for the fear of an unsafe home, and serial killers for the fear of random violence. What often happens is that a monster becomes successful and imitators replicate the trappings of the horror rather than the emotional content. So the genre has gotten a reputation as a repository for cliché.
On the other hand, it's hard to write effectively about the horror of terminal disease. I've seen a lot of horror stories about cancers that grow to world-devouring proportions, I've seen STDs turning people into vampires, I've seen people who die of wasting diseases haunting the bedrooms they've died in. Maybe it's because most horror fears are highly abstract and terminal disease is far more personal so terribly common, but most of the horror stories I've seen feel as tactlessly blatant and heavy handed as early-season Buffy The Vampire Slayer episodes.
I've written before about how my my favorite horror movies are less about abstract concepts and more about concrete circumstances in my own life. "The Disease Artist" works. Rather than exaggerating the symptoms of disease into a carnival grotesquerie, it discusses the relationship people have with sickness and mortality. It taps the universal fear of illness in a way that tells a very human story.