Did two Florida dentists who died of AIDS within months of each other infect their patients, and if so, how? In both cases, the central questions were the same.

In the first case, investigators from the Centers for Disease Control and Prevention concluded that David Acer, a dentist in Jensen Beach, Fla., somehow transmitted the virus to six of his patients during dental procedures.

In the second case, reported last week in the Annals of Internal Medicine, the CDC concluded that a dentist who practiced in an impoverished Miami neighborhood and sometimes reused disposable equipment did not infect 28 of his patients who contracted the disease. Most had sex or drug habits that put them at high risk for contracting HIV, a team of investigators concluded.

In both cases, the CDC based its findings in large part on the similarity of DNA viral sequences. In the Acer case, tests showed that the dentist and his patients were infected with very similar strains. In the second case, the unnamed Miami dentist and his patients were infected with different viral strains indicating multiple sources of infection, the agency said.

The most famous victim in the Acer case was Kimberly Bergalis, 23, a self-described virgin who, in gripping testimony delivered months before her death, blamed Acer for killing her and urged Congress to enact legislation requiring that health care workers be tested for HIV.

After extensive investigation, CDC officials said they do not know how Acer infected his patients. It remains the only documented example of HIV transmission from an infected health care worker to patients, and questions about the case and its conclusions remain. In the past year, three media reports have suggested that, in the words of Mike Wallace of the CBS news program "60 Minutes," maybe "the dentist didn't do it."

Wallace suggested that each of the six patients the CDC concluded had been infected by Acer had other risk factors for AIDS that the CDC either disregarded or never unearthed. Bergalis, in a videotaped deposition, told investigators for Acer's insurance company that she had had oral sex with a boyfriend. A doctor hired by Acer's insurer said that a physical exam found that Bergalis was infected with the human papilloma virus, a venereal wart.

The "60 Minutes" report also raised questions about CDC's reliance on DNA viral sequencing, a technique a virologist challenged as inconclusive.

CDC's AIDS chief, Harold Jaffe, wrote a letter of protest to CBS News officials. Last week he and other CDC officials took the unusual step of denouncing "60 Minutes" in a medical journal.

In a companion piece to the agency's report on the Miami dentist, Jaffe and others accused "60 Minutes" of omitting information that contradicted the program's conclusions. They also denied that CDC had not conducted a thorough epidemiological investigation and defended viral sequencing as a reliable technique. That view is supported in an editorial about DNA sequencing of HIV, written by Gerald Myers of the Los Alamos National Laboratory, where the DNA tests were performed.

In the Bergalis case, for example, the CDC team said that it knew about the sexual activities Bergalis described in the videotaped deposition. However, they noted, "60 Minutes" failed to disclose that Bergalis's two boyfriends had tested negative for HIV.

Producer Josh Howard denied that the program failed to disclose relevant details. "Who knows how many boyfriends she had?" he said.