Why Is Providing Definitions of Health, or Subjective Experience of Symptoms

Why is providing definitions of health, or subjective experience of symptoms, comparing populations to determine the levels of inequality between them often difficult?

A) Record keeping is often inaccurate and sporadic in some populations.
B) Health is defined by some medical professionals only by symptoms.
C) Different populations can have very different health belief systems.
D) Definitions of health vary among medical professionals.