Kitsunetsuki is a Nogitsune's ability to specifically possess and control others to carry out the dark spirit's bidding. It literally translates to "possession by a Fox spirit." Another aspect of kitsunetsuki is to copy and assume the shape of its host (a form of shapeshifting).
If a Fox Spirit is able to possess a human host, and take on their human shape, this means the Fox is over a 100 years old.
On Teen Wolf, "You can't be a Fox and a Wolf." Due to this, werewolves or Wolf-based creatures/spirits are immune to Kitsunetsuki.
The control of a Nogitsune spirit can be countered by the herb letharia vulpina. Poisoning by the herb will break the Nogitsune hold over said host temporarily. Nogitsune spirits can be completely exorcised from their host, or their host-copy shapes can be killed, by two known methods. One is changing the body of the host. Such as being changed by the the Bite of an Alpha werewolf because of said rule above. The other method is being slain by a weapon wielded by a Kitsune.