No one knows just what to make of the phenomenon, but health officials fear it could signal the emergence of a new AIDS virus-an agent that acts like HIV but is different enough to escape detection by any available blood test. That’s not a happy prospect. HIV screening has played a crucial role in AIDS prevention since the first tests were developed seven years ago. If the new cases are the work of a new virus, there is currently no way to track it in the population or the blood supply. As Dr. Michael Merson, director of the World Health Organization’s Global Programme on AIDS, told NEWSWEEK, “We need to get to the bottom of this.”

So far, doctors have identified roughly a dozen such cases. Dr. Bijan Safai of New York’s Memorial Sloan-Kettering Cancer Center described one during a presentation at last year’s International Conference on AIDS. The patient was a New York City man who acknowledged having AIDS-afflicted sexual partners. His illness started with Kaposi’s sarcoma, an AIDS-related cancer. Within two years he suffered a massive loss of CD4 cells, developed an AIDS-related pneumonia and died.

This spring, researchers from Spain and the United States described a similar case in a letter to the British medical journal The Lancet. The patient, a married man from Spain with no acknowledged HIV risk factors, has suffered both Kaposi’s sarcoma and tuberculosis since 1990. His CD4 count has dropped from 120 to 40 (the normal range is 600 to 1,200). Yet after two years of repeated testing, scientists at the U.S. government’s Center for Biologics Evaluation have found no sign of HIV in his blood. Dr. Vincent Soriano, one of the Spanish researchers studying the case, says his group is now following three other patients who have “borderline” CD4 counts and at least one AIDS-related illness. All three have HIV risk factors, but none has HIV.

Nor does any of the six American patients identified by epidemiologist Thomas Spira of the U.S. Centers for Disease Control (CDC). In a recently published abstract, Spira reports that three of the six have histories of blood transfusion (two received blood only after blood banks started screening for HIV). Of the remaining three, one has used IV drugs and one is a health-care worker. Spira deduces that “HIV may not be the only infectious cause of immunosuppression in man.”

He’s not alone in that suspicion. In New York and southern California , researchers are not only identifying HIV-negative AIDS cases but racing to link them to an HIV-like virus. Teams led by Dr. Jeffrey Laurence of New York’s Cornell University Medical College and by Dr. Sudhir Gupta of the University of California, Irvine, have submitted evidence to scientific journals. Neither group is publicly discussing its findings until they’re published.

Some experts caution that a new virus is not the only possible explanation. Dr. Alvin Friedman-Kien of the New York University Medical Center notes that gay men and IV drug users contract numerous infections, from gonorrhea to herpes and hepatitis. Some ostensible AIDS cases may simply reflect the immune-suppressing effects of common germs or of poor nutrition, he says. Laurence counters that when common infections suppress the immune system, the effect isn’t normally chronic and progressive. And while poor nutrition may dampen one’s overall defenses, it shouldn’t selectively obliterate CD4 cells.

The prospect of an infectious agent is no cause for panic. If there is a new AIDS virus, it doesn’t appear any more contagious than HIV. Some of the stricken patients may deny having HIV risk factors, but there’s no evidence that they have contracted, or transmitted, their illness through casual contact. The safeguards that block transmission of the known AIDS viruses (HIV-1 and HIV-2) would presumably protect people from a related agent. The big question is how to protect the blood supply from a contaminant that isn’t yet detectable. “If a new agent were identified,” says Dr. Harold Jaffe, acting director of the CDC’s AIDS division, “developing a blood test would be critical.” On that score, one can only be glad that the new disease–whatever its cause-is still so rare.