Mary's situation was extremely special.
Although the plague didn't hurt her and could control her emotions, it seemed to have a mind of its own. It kept trying to break free from the body and fought against Mary every day, but Mary didn't realize it. It could only be said that the plague was quite cunning.
It was only after grace and Helen's inspection that they discovered this.
Grace had a rich experience of "seeking common ground while reserving differences" with plagues, so she had a deep understanding of them.
In the end, Helen created an armor to restrain pestilence. The purpose was to use Mary's own willpower and mental strength to interfere. After the jacket was strengthened, it was used to restrain pestilence in all directions, giving him the illusion that he was always on alert, and thus not daring to make any moves.