Casting process is verbal.
Charisma, Wisdom and Air doctrine help in casting this spell.
Primary: wisdom Secondary: constitution Tertiary: charisma
While lesser purgation is able to cure almost all from common ailments to black plague it doesn't really affect magical diseases in anyway. Its stronger big brother, purgation, on the other hand has been especially developed by the air priests to attack the effects of this abominable magical diseases that can be extremely dangerous and wide spread. While not always succesful in curing them it at least lessens their effects.