Rough lore for the moment, tw for weird:
||Sans is the sole living being in his AU, Alphys had been experimenting in her lab and caused a parasite that infected her, she started losing her 'scales' and she was completely blinded; It spread and Sans, solely due to main character logic, was immune to it, he's barricaded in his house with a deformed Papyrus.
Papyrus is the only infected that's not aggressive, Sans smokes to de-stress himself and sometimes goes out hunting, still being wary of who his friends, as they once were, turned into.
Sometimes knocking is heard on the windows or the front door, he never opens.
These parasites substitute the organs of the subjects and take their place, creating the perfect mindless drones at their service, to recognize what organ they substitute they have different traits and shapes.
These maggots are able to create visual hallucinations to lure their preys in, as an example, Papyrus thought an hallucination was Sans feeling down, got too close and got infected.||
I'd have to think about Napstablock and MTT since they're not really uhh able to get sick or stuff like that so.