It is a result of the reductionist view of nature as divisible and summable. The sum of the parts, alas, is not always the whole and there is no such thing as an infinite set of the rules of nature, only an asymptotic approximation of it. The distinction between the patient and the outside world is superfluous and wrong. The patient AND his environment are ONE and the same. Disease is a perturbation in the operation and management of the complex ecosystem known as patient-world.
Humans absorb their environment and feed it in equal measures.
This on-going interaction IS the patient. We cannot exist without the intake of water, air, visual stimuli and food. Our environment is defined by our actions and output, physical and mental. Aging and deformities also belong in this category. But this, again, is a counter-productive approach.