But over time, his life came to be fraught with ethical dilemmas. For instance, what if a mature David at some point demanded freedom from his hermetic environment, regardless of the risk? A psychologist who worked with him has recounted how the boy, always cheerful in televised images, sometimes raged off-camera at the terrible hand fate had dealt him.

In October 1983, his doctors tried a new bone marrow technique, one that did not require a perfect blood match. As had originally been planned, Katherine Vetter was the donor. At first, the procedure seemed to work. But dormant and undetected in Katherine’s marrow was a virus, Epstein-Barr. It proved to be a killer, the trigger of cancerous tumors that overwhelmed David’s body.

At long last, he was taken out of the bubble and treated in a sterile hospital room. For the first time in his life, he was able to receive that most primal of human contacts: a mother’s kiss. But the end was at hand. On Feb. 22, 1984, two weeks after leaving the bubble — with a wink to his doctor, William T. Shearer, as a final gesture — David died.

Since then, medical science has progressed to a point where a bone-marrow transplant is usually successful in treating SCID when it is done within a baby’s first three months. Early SCID detection — it can be done in utero, the Retro Report video shows — has also greatly improved.

A possible new weapon in the medical arsenal is gene therapy, still in the clinical trial stage. With a harmless virus serving as the carrier, a healthy gene is inserted into a patient’s system to do the work of a defective gene that is the source of diseases like immune deficiency, sickle cell anemia and hemophilia. Gene therapy’s healing potential is widely accepted. But it has had plenty of setbacks along with triumphs since it was first tried in 1990, and has yet to be blessed by the Food and Drug Administration. Inevitably, too, there are ethical considerations. For some people, anything that smacks of genetic tinkering can touch off Frankenstein fears about whether we are redesigning human beings.

Nor have doctors been let off the hook. The Rev. Raymond J. Lawrence, who was the director of clinical pastoral education at Texas Children’s Hospital during David’s time there, spoke at a 1975 conference that focused on the ethics of the boy’s medical care. Looking ahead to when David might reach his teens, Mr. Lawrence asked, “Could this person live for 15 years in that kind of isolation and be human?” Doctors at the conference generally responded that their obligation was to do what they could to preserve the boy’s life until they might be in a position to make it normal.

David was the last person believed to have been placed in so confined a bubble (other than in a few weak stabs at humor in film and on television). He left a legacy for science that included a new understanding of the role viruses can play in causing cancer. “What he gave us was a powerful lesson in many areas of medicine — and just in life itself,” Dr. Shearer told Retro Report.

The boy was buried in Conroe, a town he never truly knew. In noting that “he never touched the world,” his epitaph added, “But the world was touched by him.”