FOR three years, medical sleuths have been trying to figure out how Dr. David J. Acer, a Florida dentist, infected six of his patients with the AIDS virus. But they are stumped, and the case has become one of the most disturbing unsolved mysteries in the annals of medicine.

Now, some are asking whether it should be considered a murder mystery.

The late Dr. Acer is the only health care worker anywhere known to have infected even one of his patients. Last month, a teen-aged girl became his sixth patient to test positive for the same strain of the virus that killed him. Fifty-seven other health-care professionals have told the authorities that they are H.I.V.-positive; 19,000 of their patients have been tested. Not one has caught the virus from medical treatment.

Because of publicity about Dr. Acer and his patient Kimberly Bergalis, who died of AIDS in 1991, legislators and public health officials have come under enormous pressure to restrict infected health-care workers.

Before Miss Bergalis died at age 23, she and her family waged a crusade for laws mandating that all medical workers get AIDS tests and be forced to tell their patients if they were infected. Her father, George, argued that "someone who has AIDS and continues to practice is nothing better than a murderer."