the Darkblight is a supernatural infection infused with the energies of both the First World and Jeharlu.
How is it possible to create an infection or supernatural infection ?
With Craft (alchemy) or/and Heal skills ?
Thanks for your future help.
Magic is currently the only available means to cause an infection as far as I know. And it typically uses stats for already existing diseases, the spell simply causes them to contract it (and possibly increases the rate at which it causes them to make saves or take the damage).
As far as I know, there is no method to create a custom plague, and as someone who has GM I wouldn't want there to be.