I think this has a lot of potential to be a great little "well, that's going to be a fun series of nightmares…" SCP. It's pretty simple and straightforward, but the "loosely based on things that actually happen combined into something worse than either" carries it a long way. Where you need to shore this up is on your tone and presentation.
The writing isn't bad, but clinical phrasing isn't just about keeping with the style of the site. One of the core things that makes the site work so well is that a dry clinical description can amplify the creepyness of an idea exponentially. Especially with body horror, the more detached you sound about it, and the more you break up the shocks, the worse(aka: better) it gets. You can (and probably should) play that for all its worth here.
This is just a "for example" rework of the opening lines of the first paragraph.
Original:
SCP-YYY is a novel strain of Devil Facial Tumor Disease. The disease is an infectious cancer which uses a rare method of contagion—the cancer cells themselves are the vector for the disease. The disease is endemic in Tasmanian devils, where it is passed between devils through introduction of infected tissue in lesions, usually coming from the mating behavior of the devils, which involves biting each other's face.
My hypothetical rework:
SCP-YYY is a novel strain of Devil Facial Tumor Disease, an infectious cancer endemic to the Tasmanian Devil. Both SCP-YYY and the standard form of the disease are spread via the introduction of infected tissue to lesions in the skin.
Basically, streamline as much as possible. Try to keep a logical flow of thought without too many back and forth topic switches, use footnotes when you want to interject something that isn't directly important but is worth mentioning at that time. At the same time, drop subtle hints like the "in the original host species" in my example rework to help build up the various reveals along the way.
I'd also suggest reading what you write out loud to make sure it sounds as good as it did in your head, because it usually won't on the first few attempts.(It really does help a lot.)
I honestly think that while this one might not get too much love from the "everything needs story" crowd, its a simple, solid, weirdly plausible idea that really doesn't need too much dressing up to make it work, just a polished delivery.
