[Kromm]

If a player came to me with this, I'd probably rule as follows: "The pathogen is gone, ending future fatigue, injury, and symptoms, but there would be no effect on existing fatigue, injury, and symptoms."

To me, Immunity to Disease for a living organism suggests an immune system capable of stopping pathogens cold. If someone suddenly gained that amazing capability, I'd be fine with letting it work in minutes. Unrealistic? Being totally immune isn't realistic, so we have no idea how such an immune system would work. A weird ability that makes somebody totally immune is if anything less realistic, and might well be explained as "summoning a perfect immune response at the site of each and every hostile microbe in their body."

Is this better than Healing (Disease Only, -40%) [18]? Sort of. Healing has the scary IQ penalties and FP costs . . . but it also cures the disease in full. You end up symptom-free and healthy, because Healing regenerates the body. Affliction 1 (HT; Advantage, Immunity to Disease, +100%) [20] costs only a bit more and evades the huge penalties and costs, but I wouldn't let it do anything about symptoms – the sick person would still be sick, just not diseased. You'd still need something like Healing (Cure Affliction, +60%) [48] or Healing (Affliction Only, -40%) [18] to clean up after it.

If that seems too generous, I'd suggest simply tacking on Cosmic, Lingering special effect, +100% – or even Extended Duration, Permanent, +150%, and declaring the "reasonable set of conditions that will dispel the effect" to be the end of the disease (which happens directly). That would add 10-15 points to the Affliction cost, which I think is more than enough for game balance.