why it is natural for them and not for MC and Cisney?
Maybe because their sexual desire is stronger than the others. The grandma did say that men suffer more since they can even die by this desease. Maybe Cisney is a special case among women in which she has stronger sexual disires because of how horny she gets. In every case of some desease there are people that have stronger symptoms because of gender, age or their imune system.
What does MC mean by this?
I guess when Cisney got that desease she already was in an age where she already knew about her body and went to the doctor. The mom probably didn't and just thought this was the normal case of puberty and that every woman gets this horny.
So Ruby is young now and has not experienced this disease yet?
Probably. Like I said with the mom, maybe there is some certain age to that and the mother was an exception to this. It'S also possible that Ruby just did not tell anybody, I mean we can see how much she wants to get into her cousins pants.