Medicinal Purposes : HEALTH AND HAPPINESS <i> By Diane Johnson (Alfred A. Knopf: $19.95; 255 pp.; 0-394-58717-0) </i>

Whatsoever house I enter, there I will go for the benefit of the sick, refraining from all wrongdoing or corruption, and especially from any act of seduction of male or female, of bond or free.

So reads the Hippocratic oath, written in the 4th Century BC as a code of behavior for physicians. But 24 centuries is a long, long time. These days, the relationship between a male doctor and his female patient has become an erotic time bomb, ticking away in the territory between professional ethics and the inevitable interactions between a man wearing the white coat of authority and a naked woman in need.

Diane Johnson, an experienced writer whose readable, sparkling prose is combined with an intense moral seriousness, has taken on the exploration of this territory in her novel “Health and Happiness.” Johnson is a master storyteller, and the issues that the book addresses, issues arising out of a situation in which individual men have the power of life and death over other men--and women--are gracefully manifested in characters who intersect with each other’s lives in a fascinating and satisfying narrative.

The book’s heroine is Ivy Tarro, the nursing single-mother of an infant daughter. Ivy lives in a spare San Francisco apartment with a great view, and works in a trendy restaurant. Her swollen arm, which turns out to be nothing more than a clogged mammary gland, is misdiagnosed by her own doctor and then by Dr. Bradford Evans of Alta Buena Hospital as a serious axillary vein thrombosis. Because of the misdiagnosis, she is treated with the wrong drug, and when she reacts badly she is given more of it.

During the three weeks Ivy spends in the hospital, while she sickens, almost dies and recovers, the life of the hospital becomes more real to her than her real life. There she meets Mimi Franklin, the good-hearted paid volunteer whose serenity has been shattered because the hospital wants to buy her house in order to tear it down and expand parking facilities. Mimi has had a disastrous affair with Dr. Evans, and like many women at the hospital, she has a crush on the handsome, powerful Dr. Philip Watts, who is as famous for his perfect marriage to a blond heiress named Jennifer as he is for his grace under pressure. “It would be easy enough to think herself a little in love with Philip Watts,” Mimi muses. “Such a bundle of manly qualities!”

But Philip Watts, the man who has his life and his career completely and satisfyingly under control, is not destined to have a comfortable affair with Mimi Franklin. He is destined to fall hopelessly, totally in love with the flaky, irrational Ivy Tarro, a woman whose head never has a chance against her heart. “She was a young red-headed woman of exceptional beauty,” Dr. Philip Watts notes in his last dispassionate observation of Ivy. “This he noticed in spite of himself--the cloud of red hair, the camellia-like pallor. From this distance he could not see the condition of her arm. But her beauty claimed his notice as a loud noise might, like someone dropping a bottle behind you, or a truck backfiring, the sound impinging on the consciousness of an unwilling man who was thinking about something else.”

As Johnson’s narrative seamlessly shifts from Philip’s point of view to Ivy’s and Mimi’s and back again, each of their lives is changed. First the imperturbable Philip Watts finds himself blowing up at another doctor about the way he’s dressed. The change climaxes when Watts, always a stickler for protocol, finds himself storming into the operating theater to rescue Ivy from the surgeons’ knives. In doing so, he almost hits the surgeon.

For a conventional doctor, this kind of intervention with colleagues is a violation of all hospital procedures. But Philip Watts is no longer a conventional doctor, he is a man in love. As he looks down at Ivy Tarro, he changes. “Her face, even with the calm of anesthesia, bore traces of anguish. He thought he could discern a tear stain, a dampness at the hairline where her head was wrapped in a bandage of sterile cloth. He was moved in a way that he was at a loss to put a name to. Relief and success were surely part of it--he had frustrated the surgeons and rescued the young woman. . . . He had a strong impulse to pick her up and carry her in his arms to safety, like a fireman.”

Each character in the book is perfectly drawn, from the ditzy Jennifer, who agonizes about the morality of eating shellfish because it may have a high pain threshold, to the threatening bureaucrat from Alta Buena who tries to bully Mimi into selling her house, to the malicious Chinese exchange student. Johnson gets the way doctors treat each other--their rivalries, comradeships and indecipherable language--just right: “Dr. Evans made the diagnosis of a blood clot of unknown origin obstructing the subclavian vein, and this was substantiated by a venogram,” one doctor explains to another.

Ivy herself is an enchanting and sensitive character. She wakes up from her fever one night with the sense that someone is hiding in the room she shares with another patient. “Death in the room. Ivy could feel his gaze on her too,” she thinks in a panic. “A choking sensation in her throat became so intense that she was obliged to struggle to sit up a little. She opened her eyes. She stared at the hem of the dividing curtain, expecting death’s shoes and pantlegs to be visible as he quietly stood over her friend in the other bed.”

This is not a novel about death but about life, and about the sneaky way that love and life conspire to triumph over death, even in a hospital, even with doctors and their patients.