Pope Francis addressed a number of the leading cancer researchers in the world, assembled at the Vatican for a conference on regenerative medicine, including immunology. Photograph by Giuseppe Ciccia / NurPhoto via Getty

Late last month, a number of the leading cancer researchers in the world assembled at the Vatican for its third conference in regenerative medicine. The event, titled “Cellular Horizons,” focussed in part on immunotherapy, a form of treatment that is intended to boost the body’s innate ability to fight off tumors. It included an address by Pope Francis, who stepped down from the stage in the cavernous Paul VI Audience Hall to personally greet many of the attendees, and ended with a private performance by U2’s the Edge inside the Sistine Chapel. That the researchers found themselves treated as dignitaries must have been a little disorienting. Although immunotherapy is now widely regarded as among the most promising avenues in cancer treatment, until recently it was known primarily for its long history of failure. During a panel discussion at the Vatican, James P. Allison, the discoverer of a new class of immunotherapy drugs known as checkpoint inhibitors, noted that for a long time his fellow-researchers were looked upon as “snake-oil salesmen.”

The Vatican might seem an unlikely place for a medical conference, but in this case it was fitting. The thread that likely connects Catholicism with immunotherapy runs all the way back to February 22, 1952, when a twenty-nine-year-old housewife in Baltimore noticed that her four-year-old daughter, Ann O’Neill, was pale and feverish, her neck mottled with blue. Only a few days later, doctors at St. Agnes Hospital had diagnosed the girl with acute lymphatic leukemia, a type of cancer that impedes normal white-blood-cell production. Several months later, after a brief partial remission, Ann was placed in an oxygen tent and given no more than a day to live. As her mother told a reporter in 1963, the girl “had lost all spirit: it was almost as if life were gone and just some reflexes were left.”

St. Agnes was founded in 1862 by the American branch of the Daughters of Charity, an order dedicated to ministering to the sick and poor. As Ann lay dying, the head pediatrics nurse, Sister Mary Alice Fowler, approached her parents and suggested that Elizabeth Seton, the order’s late founder, could perhaps intervene on Ann’s behalf. Although Seton had died in 1821, the sisters at St. Agnes had good reason to turn to her. In 1935, Seton had been credited with curing a nun in New Orleans of pancreatic cancer. Now she needed a second miracle to continue her journey to sainthood.

Ann’s parents agreed to Fowler’s plan for a novena—nine days of prayer. A small piece of cloth that had touched Seton’s remains was affixed to Ann’s nightgown, and, according to some versions of the story, the girl was also taken to visit Seton’s tomb. Her recovery began almost immediately. Within two weeks, she was released from the hospital with no trace of leukemia in her blood. Two years later, with Ann still in perfect health, the O’Neills received a visit from a Vatican investigator, known as a devil’s advocate, whose job was to search for evidence that could explain Ann’s recovery by non-miraculous means. After an eight-year inquiry, which included a thorough review of Ann’s case by Sidney Farber, the man who pioneered the first effective treatments for leukemia, the verdict came down from the Vatican: it was a miracle. Seton was canonized in 1975, the first native-born American to become a saint.

The Vatican’s medical investigation was, by all accounts, exhaustive. Milton Sacks, a prominent hematologist at the University of Maryland Hospital who helped treat Ann, testified that he had never seen or heard of a similar recovery. And yet the investigation did not, ultimately, give credence to one possible explanation for the girl’s dramatic change of fortune. Shortly before her leukemia went into complete remission, raw blisters broke out across her body. Sacks would later describe it as among the worst cases of chicken pox that he had ever seen.

Was it possible that, in attacking the virus, Ann’s immune system had killed the leukemia, too? This sort of phenomenon had been recognized since at least the late nineteenth century, when William Coley, a surgeon in New York, discovered that a rapidly growing sarcoma had vanished from the neck of a German immigrant after he contracted a serious skin infection. In the ensuing years, Coley attempted to treat many other cancer patients with a bacteria-filled concoction that became known as Coley’s toxins. But neither he nor his successors ever had much success, and for most of the twentieth century immunotherapy remained at the fringes of cancer research—which is perhaps why the devil’s advocate ruled out the chicken-pox cure. In the past five years, though, the field has once again taken on an aura of promise. Last month, Sean Parker, the billionaire co-founder of Napster and former president of Facebook, announced that he was launching his own institute for the study of immunotherapy. Jeffrey Bluestone, the institute’s C.E.O., told me in Rome that this is an “incredible time” for cancer research.

Looking back at Ann O’Neill’s case now, an immune response to chicken pox seems a plausible explanation for the disappearance of her cancer. “For me, that’s endogenous immunotherapy,” Chi Van Dang, the director of the Abramson Cancer Center at the University of Pennsylvania, told me. “Without any proof at all, it’s almost certain that’s what happened.” And it’s possible that other Vatican-sanctioned cancer miracles are due to an immune response, too. In 2014, for instance, an article in the journal BMC Cancer noted that St. Peregrine, the patron saint of cancer patients, who died in 1345, likely had an infection before his own miraculous recovery from cancer—though the story may be apocryphal. (A book on tumors that disappear spontaneously, published in 1966, suggested that they be called “St. Peregrine Tumors.”)

Neither Seton nor Peregrine seems to be at any risk of losing their sainthood. For all the current enthusiasm, immunotherapy has a long way to go. Tak Mak, a Canadian researcher who, in 1984, discovered the receptor that allows the immune system to recognize some of its foes, reminded the Vatican’s conference-goers that attacking solid tumors remains a major hurdle for the field. Such tumors are protected by a coating of cells known as macrophages, which keep the immune system’s main assassins, the T-cells, from entering. “It’s almost as if this layer of macrophages is the wall that separates the Greek soldiers from getting to Helen of Troy,” Mak said. He added that he and his colleagues are now searching for the microscopic Trojan horse that can break through the wall. If immunotherapy researchers can find a way to do that, we may have a cancer miracle that everyone can believe in.