Demons. Shinigami are warned to stay away from them in their early days of the academy.

"Demons are foul creatures made of sin and debauchery."

They are loathsome creatures of sin, debauchery, and filth. They are a curse on the world.

"They are not to be trusted, or allowed to live. They steal what is important to us."

To Shinigami they are considered low and foul, creatures living and breeding in filth.

"Never let a demon touch you. Never let it try and seduce you. It only has one goal in mind."

It isn't always because demons seek to feed on what all Shinigami prize above all else.

"Corrupting the pure to devour its soul."

Souls and their own sanity. Demons devour a Shinigami in his own desire.

"They will do anything in their power to persuade you to let them continue to exist."

Demons are impossible to resist, their touch incites fire in even the most pious of Shinigami.

"Don't allow yourself to be tempted by these noxious creatures, William."

…I regret to say that it's a suprisingly common problem.

"Demon Addicted shinigami are ill shinigami, William. Never let them seduce you."

They become the willing pets of demons like Sebastian Michaelis. Shinigami like William T. Spears of Dispatch Management.

"…Because a shinigami owned by a demon is on a chain… On which he can never break."