The Kaibutsu are the white blood cells and the red demons are the virus. When you face them before the Rust stage, they only have the power to deactivate your current effect. After that, however, they have the power to kill you and even infect Kaibutsu (as of 0.15)
And much like white blood cells, the standard Kaibutsu also eliminates foreign threats to their land, which happens to include Sabi because she does not belong there.
If you're looking at it from a cellular standpoint, anyway.