True, my point was that the disease did a lot more harm in the new area than in the old, smallpox killed numerous europeans, however, it killed a lot more amerindians (relatively speaking of course).
So while the commen cold might be well known on Scadrial and/or Sell, it was not known on Roshar (or at least in the Purelake), because of that, a lot of people die of it, and it can be called a plague. Whether the worldhoppers knew this would happen is an entirally different question, as is the influence of stormlight on disease.
I do believe that stormlight has a positive effect, and less people die of the unknown disease than would on, for example, Scadrial, where there probably isn't such an effect.