The principle of the Form is Platonic. That should suffice, but many do not have exposure to the Platonic concept of Form.
Plato held that individuals are merely representations of an Idea, a model of something which is intangible, but to which the individuals resemble to a greater or lesser degree. Individuals can be said to approximate or attain perfection, to their identity withing the form, and deviate from perfection as they deviate from the perfect Idea.
The point is not offered for amusement. Human wellness is a Form. Human disease is not. Disease, as will be discussed later, is a perturbation from that Form or Idea of the Human in Wellness.