It is, in a very literal sense, demonic possession, witchcraft and/or sorcery (verbal and social contagions - "spells").

Do you notice any coerced mass-initiations into alliances over filiations nowadays? ("pacts" over "families"?):

Sorcerers have always held the anomalous position, at the edge of the fields or woods. They haunt the fringes. They are at the borderline of the village, or between villages. The important thing is their affinity with alliance, with the pact, which gives them a status opposed to that of filiation. The relation with the anomalous is one of alliance. The sorcerer has a relation of alliance with the demon as the power of the anomalous. The old-time theologians drew a clear distinction between two kinds of curses against sexuality. The first concerns sexuality as a process of filiation transmitting the original sin. But the second concerns it as a power of alliance inspiring illicit unions or abominable loves. This differs significantly from the first in that it tends to prevent procreation; since the demon does not himself have the ability to procreate, he must adopt indirect means (for example, being the female succubus of a man and then becoming the male incubus of a woman, to whom he transmits the man's semen). It is true that the relations between alliance and filiation come to be regulated by laws of marriage, but even then alliance retains a dangerous and contagious power. Leach was able to demonstrate that despite all the exceptions that seemingly disprove the rule, the sorcerer belongs first of all to a group united to the group over which he or she exercises influence only by alliance: thus in a matrilineal group we look to the father's side for the sorcerer or witch. And there is an entire evolution of sorcery depending on whether the relation of alliance acquires permanence or assumes political weight. In order to produce werewolves in your own family it is not enough to resemble a wolf, or to live like a wolf: the pact with the Devil must be coupled with an alliance with another family, and it is the return of this alliance to the first family, the reaction of this alliance on the first family, that produces werewolves by feedback effect. A fine tale by Erckmann and Chatrian, Hugues-le-loup, assembles the traditions concerning this complex situation.

The contradiction between the two themes, "contagion through the animal as pack," and "pact with the anomalous as exceptional being," is progressively fading. It is with good reason that Leach links the two concepts of alliance and contagion, pact and epidemic. Analyzing Kachin sorcery, he writes: "Witch influence was thought to be transmitted in the food that the women prepared. . . . Kachin witchcraft is contagious rather than hereditary ... it is associated with affinity, not filiation." Alliance or the pact is the form of expression for an infection or epidemic constituting the form of content. In sorcery, blood is of the order of contagion and alliance. It can be said that becoming-animal is an affair of sorcery because (1) it implies an initial relation of alliance with a demon; (2) the demon functions as the borderline of an animal pack, into which the human being passes or in which his or her becoming takes place, by contagion; (3) this becoming itself implies a second alliance, with another human group; (4) this new borderline between the two groups guides the contagion of animal and human being within the pack. There is an entire politics of becomings-animal, as well as a politics of sorcery, which is elaborated in assemblages that are neither those of the family nor of religion nor of the State. Instead, they express minoritarian groups, or groups that are oppressed, prohibited, in revolt, or always on the fringe of recognized institutions, groups all the more secret for being extrinsic, in other words, anomic. If becoming-animal takes the form of a Temptation, and of monsters aroused in the imagination by the demon, it is because it is accompanied, at its origin as in its undertaking, by a rupture with the central institutions that have established themselves or seek to become established.

Let us cite pell-mell, not as mixes to be made, but as different cases to be studied: becomings-animal in the war machine, wildmen of all kinds (the war machine indeed comes from without, it is extrinsic to the State, which treats the warrior as an anomalous power); becomings-animal in crime societies, leopard-men, crocodile-men (when the State prohibits tribal and local wars); becomings-animal in riot groups (when the Church and State are faced with peasant movements containing a sorcery component, which they repress by setting up a whole trial and legal system designed to expose pacts with the Devil); becomings-animal in asceticism groups, the grazing anchorite or wild-beast anchorite (the asceticism machine is in an anomalous position, on a line of flight, off to the side of the Church, and disputes the Church's pretension to set itself up as an imperial institution); becomings-animal in societies practicing sexual initiation of the "sacred deflowerer" type, wolf-men, goat-men, etc. (who claim an Alliance superior and exterior to the order of families; families have to win from them the right to regulate their own alliances, to determine them according to relations of complementary lines of descent, and to domesticate this unbridled power of alliance).

— Deleuze & Guattari, A Thousand Plateaus, 1987