Question about the virus (Possible spoilers)

Okay, so I understand that the Medusa virus wasn't in fact a virus at all, but something from outer space. I'm just a little confused as to why some of the people, like the girl Alice and the protagonist's twin sister, didn't turn into stone while others did?

I read something about the virus being able to bring imagination to reality, but I'm still confused as to why others turned to stone? Did they just not have strong enough imaginations or something? I feel like that last one is a stupid question, but it's a question that is bugging the hell out of me! If someone could explain, even just a little bit, I'd feel very grateful!