A Faust-Class Physician : THE M.D.: A Horror Story <i> By Thomas M. Disch</i> ; <i> (Alfred A. Knopf: $22; 401 pp.) </i>

It would be easy to make this book seem unreadable by praising it. Thomas M. Disch’s “The M.D.,” we might say, is a fine and scathing anatomy of post-Vietnam America, a sly indictment of the organized religions of the Western world, a tragic tale of moral paralysis and sin. It depicts through scorching flashes of detail the inexorable growth of AIDS and of its even more terrible successor; it is grimly cynical about priests and politicians, doctors and fund-raisers, grannies and dads, moms and sons and daughters.

But then, after frightening any reader half to death, it would be only fair to say one more thing about Disch’s magnum opus. “The M.D.” may be profound and dark and very dire, but it is also a page-turner. And each new page, like an electric eel, is poised to shock.

In the mid-1970s, in the tangled working-class Catholic heart of St. Paul, Minn., young Billy Michaels throws a kindergarten tantrum when the appalling Sister Symphorosa insists to her captive audience of tiny children that there is no Santa Claus, and tells them why it is blasphemous to believe in him. As she strangles their joy she smiles, sure that her victims, coiled in guilt, are now ready to spend the rest of their lives negotiating with the church for salvation.

But Billy knows better. Santa Claus has spoken to him already, in a voice like gravel, out of an icy whirlwind. Santa’s other name is Mercury, and if Billy is good he will give him a caduceus for his very own, and he will be able to do things with it. He will be able to heal, and to curse. He only has to remember that nothing he does with the caduceus can be undone, and that every time he uses it for good he must recharge the two interwoven snakelike sticks by doing harm. It’s easy. Life, after all, as Billy already has learned from Sister Symphorosa, is something you can make book on; it is a series of negotiations with a carnival deity.

Besides, he has every intention of doing as much good as possible, especially when it’s a member of his own convoluted family who needs help; and if he must do evil--if for instance he must punish cigarette smokers by inflicting cancer on anyone who uses a lighter he has exposed to the caduceus--then so be it. Good, he knows, is best if done at home (it’s like giving something to yourself in secret), and evil is best done at a distance (as in Vietnam).

Coiled within the acts of the caduceus, Billy grows into William. He begins (like his country, perhaps) to live on credit, on promissory notes. But he fails to ask when his debts will come due, or in what currency he will be required to pay.

We jump to 1999. Minnesota is becoming a dust bowl, due to the greenhouse effect. Brilliant young Dr. Michaels needs a very great amount of caduceus energy to fund his good works and increase his fortune, so he creates ARVIDS--Acute Random-Vector Immune Disorder Syndrome--as a curse upon innumerable Americans he hopes never to meet.

Concentration camps begin to proliferate, some of which Michaels’ corporations run for profit. But anyone can catch ARVIDS simply by breathing: The world is rotting. Like any human being who has surrendered his mind to a coercive faith, like any citizen of a world power living in dream-time as history begins to bite, William Michaels has lost track of his soul, and the bargain he struck with the god has become a universal contagion.

Mercury, or Satan, or the vile unconscious of the race, is well satisfied. All he had ever required was that his victim operate the caduceus according to contract. That was all. He now abandons him to his fate, for he is used up.

In the last pages of Disch’s superb saga of moral death and blank-faced retribution, Dr. Michaels gets everything he deserves, in scenes of chilled pathos and steely hilarity that tickle like scalpels. Just as in any fairy tale that warns us about making bargains with the devil, he loses everything. He is damned. But because he has been corroded from within, he never understands why. He is denied even that.

In some of his best previous novels--like “334” (1972) or “The Businessman” (1984)--Disch occasionally indulged himself in the habit of creating characters visibly less intelligent than he was, incarcerating them in cages of irony, and then gloating at their failure to work out the awfulness of the trap he’d put them in.

What makes “The M.D.” so memorable a book is that almost none of its characters--not even Dr. Michaels at the well-deserved end of his tether--are treated to one instant of contempt from the author. Young Billy is any of us. In his slow path to damnation, he follows a course that any of us might find hauntingly plausible. There are some monsters in his family, as there are in most families, and one of them becomes his nemesis. But most of his kin are lovable.

Near the end of things, Billy’s stepfather Ben, who has suffered a caduceus-inspired succession of ups and downs, contemplates the Book of Job. He notices how God proposes “the crocodile as an emblem of his own awful power,” as if that answered Job’s lamentations, and Ben decides to follow the hint. “The secret wisdom of the Book of Job,” he muses, “is that it is exciting and profitable to work for crocodiles.”

What he cannot know is that by 1999, it is too late for him or his land. He has been serving his stepson’s crocodile god for years, and it will soon cast him off. Because Ben is a real person, not a caged puppet, that death will seem tragic.

But the tragedy encompassed by “The M.D.” is of course greater than that of a single family. At the age of 50, Disch has looked upon the religion of his birth (he is clearly a lapsed Catholic) and found it identical to the corruptions of Mercury: The twinings of the caduceus are a neat symbol of the intimacy of the two faiths. He has looked upon the land of his birth (he comes from Minnesota) and found its people no more capable of understanding the roots of power than Billy Michaels was.

“The M.D.” is chock-full of jokes and ironies and story-telling genius; it is a page-turner. But one more thing can be said as well. “The M.D.” is a tract for the times.