I suspect that it's actually more humans are especially vulnerable to plague than anything else, making everyone else look resistant. Because they're vulnerable to Chaos, and Nurgle exists, so...
Though thinking on it, do we know if elves can catch human diseases and vice versa, and with other species? I doubt that matters much for the extra magical diseases like Nurgles Rot, but for the closer to mundane ones would they count as sufficiently different organisms for the diseases to not work? Considering the age and general prosperity of the elven species, they could have eradicated their worse mundane afflictions. The elf averse swamp bugs in Laurelorn come to mind.
If elves are more resistant to magical diseases than humans and have eliminated the worst elf specific non-magical plagues, it could certainly appear to be a general resistance to disease.