Curiosity re-piqued, there's a lot on the matter insofar as google is concerned. Some say it's all about inherent willpower and not any biological means. Some say immunity, others say resistance. Of all the articles, this one seemed to be the best-researched of the bunch:
Hardly canonical, but it helps paint a picture, which is what I wanted to see. I'll admit I could be wrong about immunity, but the language just leaves wiggle room.