I hate that Enterprise episode because it is such an edge case for the Prime Directive, and humanity hasn't even adopted it yet.

Even if the species didn't develop warp drive, they have had contact with other species that did. This isn't a pure, uncontaminated species that needs to be protected. Hell, I think that this species even contacts the Enterprise.

And this seems to be a trivial enough cure that this species is going to find another species to cure them. If Phlox can develop a cure in weeks on a spaceship, this seems to be trivial work for space faring civilizations.