It's extremely likely that the Celestial Dragons perform experiments on humans.
While humans or other species are enslaved, the Celestial Dragons behave like an alien species that is trying to invade them.
I think the rare diseases that people get from the Celestial Dragons are not STDs, and they aren't a result of Saturn's poison (assuming he has any!).
What it actually is, is very simple. The Celestial Dragons use various kinds of torture methods and experimentation instruments; they are deliberately giving humans rare diseases as a result of experimentation.
It's stuff like they turn their blood into other substances.