David, the 12-year-old "bubble boy," died last year because a bone marrow transplant that had held out hope for a normal life instead infected his immune-deficient body with a common virus that "ran amok," causing incurable cancer, his doctor said today.

An analysis of David's autopsy by researchers at four medical institutions produced the "first conclusive evidence of cancer developing in a human being after infection by a virus," said Dr. William T. Shearer, David's physician and a professor of pediatrics at the Baylor School of Medicine here.

Doctors have long believed that common viruses, under highly selective circumstances, can cause cancer in humans.

The analysis was published in this week's New England Journal of Medicine.

"He is the patient who has tied together infections by viruses and the development of cancer," Shearer said of his world-famous patient. The boy's last name has been withheld by hospital officials to protect his family's privacy.

David received the Epstein-Barr virus -- perhaps best known as the agent that causes infectious mononucleosis -- from the bone marrow of his older sister, Katherine. She was the donor in an October 1983 transplant operation that was supposed to trigger the growth of an immune system in David, freeing him from the latest in a series of germ-free plastic "bubbles" he had lived in since being born with severe combined immune deficiency (SCID).

The transplant never "took," and David died Feb. 22, 1984, as a result of massive bleeding from tumors that attacked his intestines, liver, lungs and brain.

More than 90 percent of adults have EB virus in a latent state in their blood cells, Shearer said, and researchers have long known that a transplant can activate it.

The procedure poses little risk for recipients whose immune systems are in order; the "T-cells" in their blood can overwhelm the invading virus. However, if recipients are immune deficient, as David was, or if they have received immuno-suppressant drugs to decrease the chance that their body will reject the transplant, the risk increases of the virus going berserk and triggering the uncontrollable growth of malignant cells.

Shearer said the findings from David's case will not alter the medical protocol for such transplants, but will provide a "fuller understanding" of the risks involved. He said approximately 10 percent of all bone marrow and organ transplant recipients develop cancer, and that "David himself signed the consent form that described that risk."

SCID afflicts one child in 100,000. David's treatment was unique -- no patient ever lived longer in a germ-free environment.

The boy, who had a high IQ, lived his last years at his suburban Houston home in a three-room plastic compartment complete with a small piano, a telephone hookup to school and an assortment of books and toys.

It was not until his dying days, when he was removed from the bubble for treatment, that David knew the touch of his mother's skin.

Babies born with SCID today are given bone marrow transplants shortly after birth, using technologies unavailable when David was born. Dozens of SCID babies have survived the transplants and developed immune systems even when no perfectly matched donor can be found.

Shearer said he does not know why David's transplant did not "take"; had it worked, he said, the boy might have developed immune agents in time to fight off the EB virus.

"I don't think I will ever know" why the transplant failed, he said. "That is one of the burdens of my life." One possibility, he said, is that, relative to body weight, David received many fewer marrow cells than an infant recipient would.