Death, Dying, and Resurrection

The infestation of corpses by Druj Nasu upon death greatly influenced Parsan burial ceremonies and funeral rites. Burial and cremation of corpses was prohibited, as such acts would defile the sacred creations of earth and fire, respectively. Towers of Silence were developed—open air, amphitheater like structures in which corpses were placed so carrion-eating birds could feed on them. Because of the waves of undeath that wracked the world and against which Parsa stood, many peoples that have survived the The Quieting have strong traditions that uphold the Ancient Parsan rights, but many have lost their knoweldge and fallen to burying dead. Traditionalists and cults that have evolved over the intervening millenia object so much to the burial of corpses that the exhumation of the dead can be regarded as meritorious.
While in the current day resurrection magic has seemingly been made inert, and the fate of the soul is unknown as Zahahk and his daeva walk the mortal world, there is a deep history of death rituals on Alam.

Sagdid

Sagdid, meaning “seen by a dog,” is a ritual that must be performed as promptly after death as possible. At death, the soul lingers around the body for three days in the ethereal plane while the gods worked out the balance between one's good and bad deeds. There is a minute here where a quick-acting Ashipu, or Cleric, can bind the souls to the body and stave of death. Then, the soul tranistions to the etheral plane and it is during this time that funerary rituals take place and stronger resurrection magic is needed to return the soul to the material world. The body is considered dirty at this time, as evil spirits, and most especially the Daeva Druj Nasu, are drawn to it and act to temp the dying soul to their realm and contamiate the body. Directly after death, as soon as the soul has left a corpse, the Mobad tell us, “Nasu rushes upon” the body, “in the shape of a raging fly, with knees and tail sticking out, droning without end.” Directly after death, as soon as the soul has left a corpse, “the druj Nasu rushes upon” the body, “in the shape of a raging fly, with knees and tail sticking out, droning without end”. As soon as Nasu takes hold of a corpse, the body instantly becomes contaminated. Nasu continues to inhabit the corpse until the sagdīd ritual is performed, during which a dog must look at the corpse, or until a carrion-eating bird or dog consumes the body in the Tower of Silence.
One practice is to call out to Sagdid a spiritual 4-eyed dog who can scare away the evil spirits. Mastiffs, are beneficial fauna of Ahura Mazda that act as protectors of humanoids and are brought to participate in the furerary rights, as it is said they can see into the ethereal plane and help the sagdid in their battles against the spirits. If the dogs shy away at any time, it means there were still evil spirits present and the dog will be led in again up to nine times until its willingness to approach the corpse means that all evil spirits have finally been fought off.
On the fourth day (or late on the third day), the soul leaves its body behind entirely and travels to Hammistagān where it passes through Maleke Khalagh's realm and memories and knoweldge is sifted and preserved before approaching Chinvat Bridge, the last barrier to the heavens. At this time the corpse can be brought to the Tower of Silence where it is prvented from contaminating the earth of the fire of Alam, and is consumed and rendered inert by the carrion feeders.

Corruption and Purification

Besides contaminating corpses upon death, Nasu can also contaminates those who interact with corpse-matter. Tradition holds that one must never carry a corpse on their own, lest Nasu’s infection transfers to them. Mara the Serene, an Ashipu (excorcist) of Marad, tells us that if one carries a corpse alone, Nasu emerge:

“from the nose … , the eye, the tongue, the sexual organs, and the hinder parts of the deceased, and rush upon the corpse bearer … [and stain him] even to the end of the nails, and they are unclean, thenceforth and require purification."

A contaminated individual is exceedingly dangerous, as they will exhibit no natural sign but can contaminate others and in death will quickly become a ghul, and can then infect myriad others as they seek to fulfill an unending hunger and consume the living.
In some cases, a living individual who has been defiled by Nasu has a chance at regaining purity, if the proper purification rites are performed. However, if the ritual is performed by an unqualified purifier, Nasu will grow stronger, and the contamination will heighten. If an Ashipu is not available to channel divine magic of the Benificent Immortals to alleviate the curse, then the contaminated must live in seclusion an enclosure where. Mara tells us,

“the ground is the cleanest and the dryest and the least passed through by flocks and herds, where they may live by the fire of Ahura Mazda, by the consecrated bundles of Baresma, and by the faithful. There, others must provide him with the coarsest food and the most worn-out clothes until he ages into an old man. Once he is elderly, the infected must be beheaded, and his corpse is offered to the vultures. At this point he is absolved by his repentance."

Extreme measures, as you can see, must be taken to prevent undeath from overtaking the living. Druj Nasu, it is said, are the daeva most responsible for the corruption of Alam.