Literally means "the state of being possessed by a fox." The victim is typically a young woman, whom the fox enters beneath her fingernails or through her breasts. In some cases, the victims' facial expressions are said to change in such a way that they resemble those of a fox. Japanese tradition holds that fox possession can cause illiterate victims to temporarily gain the ability to read.
Folklorist Lafcadio Hearn describes the condition in the first volume of his Glimpses of Unfamiliar Japan:
Strange is the madness of those into whom demon foxes enter. Sometimes they run naked shouting through the streets. Sometimes they lie down and froth at the mouth, and yelp as a fox yelps. And on some part of the body of the possessed a moving lump appears under the skin, which seems to have a life of its own. Prick it with a needle, and it glides instantly to another place. By no grasp can it be so tightly compressed by a strong hand that it will not slip from under the fingers. Possessed folk are also said to speak and write languages of which they were totally ignorant prior to possession. They eat only what foxes are believed to like — tofu, aburagé, azukimeshi, etc. — and they eat a great deal, alleging that not they, but the possessing foxes, are hungry. (Wikipedia)