It was, after all, Hippocrates who said: "For extreme illnesses, extreme treatments are most fitting." But today, extreme treatments involve using fetal tissue to stem brain damage, taking animal parts to repair broken hearts, mixing egg and sperm in a laboratory dish to create life -- treatments much more extreme than was even envisaged a decade ago.

Extreme illnesses, too, have a new definition in the 1980s. Gone from this country are the ravages of smallpox, polio and childbed fever.

Today, an extreme illness may be the rare disorder of anencephaly (being born without a brain) or the new epidemic of acquired immune deficiency syndrome (AIDS).

What's more, the notion of extreme illness may encompass the natural processes of birth and death, as medical science increasingly intervenes in mother nature's sometimes twisted course.

What is fitting?

To answer that question, a whole new medical discipline has burgeoned in this decade -- the field of bioethics.

At first the debate over "playing God" belonged to physicians, who wielded the medical power on behalf of their patients.

Then philosophers, religious leaders, ethicists and lawyers -- speaking for society at large -- joined the debate.

Now, consumers are playing an important role in trying to reconcile tomorrow's science with today's morals in the practice of medicine.

Much is changing. It used to be that ethical questions were either-or issues. To pull the plug or not to pull the plug was the central debate in the landmark Karen Ann Quinlan case in 1976. That case marked the first time the public grappled with the issue of whether life-prolonging treatment -- in her case, a respirator that kept her breathing -- should be stopped, to let the comatose patient die. In a landmark decision, the New Jersey Supreme Court ruled that the respirator could be removed; yet Quinlan lived on for more than 10 years.

Today, the search for what is "fitting" in medicine is not so much a decision between right and wrong. It is not simply a ruling for or against a particular treatment. Rather, it is likely to lead to a set of options -- none perfect -- for patients and their families as well as for doctors and hospitals. Within this framework of ethical alternatives, individual choices must be made.

As Dr. Joanne Lynn, an expert in geriatric medicine and an associate professor of health sciences at George Washington University Medical Center put it:

"Twenty years ago, we tried for categories: ordinary versus extraordinary treatment; active versus passive informed consent, terminal versus nonterminal patients, acceptable versus unacceptable care.

"But today," she said, "we no longer even trust our categories. The lines blur.

"The yes-no questions have become questions of choices."