Why do you think the 13 are immune?

My first thought was that, since the hivemind seems to be in a constant state of content and happiness, maybe Carol is immune because her body/mind is just incapable of constantly being happy. However, after we met the other immune, that doesn’t seem to be a unifying characteristic. The other immune seem pretty different from one another. There’s an elderly woman, a young mother, a hedonist, etc. none of them seem miserable or incapable of being happy, so why are these specific people immune?

I mean we’re talking about 13 people out of BILLIONS. There has to be something different about them that causes them to be immune. But what? And why?