The power of the A-virus was strong to begin with. Afterwards, the 'Stairway of the Sun' had even absorbed the mysterious extraterrestrial meteor and gobbled up Treebeard's Golden Fruits in the world of the Lord of the Rings, providing it with an enormous amount of energy and allowing it to separate into an animal form and a plant form. Finally, after the breakthrough challenge battle in this world, it also devoured one of its own kind. It had matured into a rather formidable stage by now.
Therefore, although Sheyan's 'Pestilence Monarch' was naturally weak against holy divine power, the divine power could only restrain 'Pestilence Monarch' to some extent, and was not completely immune to it!
Were priests and monks immune to sickness? At most, their chances of falling ill were smaller, and their conditions less severe. Therefore, Sheyan successfully spread his pneumonia to the Cardinal. The symptoms flared up immediately!