Rule of Thirds

I've been reading (so what else is new) and finally ran across Hippocrates rule of thirds as explained in Frances's book, and, put simply, I'm vaguely concerned.

Why thirds?

As a short explanation of the concept: Hippocrates believed that prognosis was a vital skill that physicians had to develop.  Part of prognosis was determining who could get value from care. He broke patients down into three categories: 1) those who would get better on their own, 2) those who could benefit from treatment, and 3) those who would never get better.

Presumably this set of standards was to prevent the loss of resources treating those who would eventually get better on their own, or those who would never get better.  Ideally only those who could benefit from care would receive it.  One of the questions that immediately springs to mind, among others, is "Who decides?"

In what way does bias feed into this game of thirds?  It's really a simple question, but it means a great deal to people who suffer from disease, particularly mental disease.  The bright lines between needing medical help, and too far gone to do anything for don't exist, including those between able to heal on one's own and needing medical or psychological attention.  All these decisions, to make them more difficult, exist withing the social structures that we are all very much aware of.

What's to ensure a man of African descent will be treated with equal consideration?  Or a woman?  Or a man?  Where does personal judgement conflict with an ethic of equality, particularly within a system so severely skewed by  not just personal, but societal bias?

I have too many questions.  What I don't have is answers, and maybe that's appropriate.  Having an answer, a defined system of consequences offers assurances that what one is doing is right.  It makes it easier to make decisions that affect people's lives, whether or not those systems of ethics truly reflect the world as it is.