Therianthropy

Lycanthropy is a disease passed through body fluids (saliva, blood, etc). If one succumbs to the illness instead of dying they develop the ability to transform between beast and human form. The ability takes years develop finer control and most are slain long before reaching any state of balance. While in beast form the human personality and mind is somewhat suppressed, less so the longer one has been a shifter. The change is painful, relatively slow, and awkward (as depicted in many werewolf movies). Interestingly an animal infected by a human lycanthrope can develop the same disease and switch from their natural form to human. The secondary form is always the natural form of the source if the species (victim and lycanthrope) are different, and the alternate form if the species are the same.