A few years ago, two Long Islanders with hepatitis C met in a support group and soon discovered they had something in common: both had become infected with the virus after open-heart surgery - by the same surgeon.

Public health investigators, who were looking into one of the two cases, had not asked members of the patient's surgical team whether one of them might be infected. Now they did. Eventually they determined that the surgeon, Dr. Michael Hall, was infected and that he was the inadvertent source of both patients' infections - and that of at least one other patient.

Dr. Hall was never found legally liable, and he continues to do hundreds of open-heart operations each year. His lawyer, Tony Sola, said last week that the doctor had tested negative for hepatitis C in recent years, that there were no restrictions on his practice and "that he did absolutely nothing wrong and operated in a perfectly reasonable manner."

Still, the episode was a window into a risk about which troublingly little is known: the possibility of getting a viral infection from a health care worker.

Viruses like hepatitis B, hepatitis C and H.I.V. are spread by blood-to-blood contact. Doctors, like cooks, often cut or nick themselves, and if it happens while a surgeon's hands are inside the patient's body cavity, the doctor is at risk of both picking up and passing on an infection. A survey in The New England Journal of Medicine last week reported that surgeons-in-training suffer an average of eight needle sticks in their first five years.

Despite the risk, however, there is no mandatory testing of surgeons for blood-borne viruses, and infected health care workers are not prohibited from practicing medicine or invasive surgery. Local expert panels are convened to review cases if they come up, but many surgeons simply refrain from being tested.

Where all of this leaves patients is a subject of fierce debate. Federal health officials say the risk of a health care worker's transmitting a blood-borne viral infection to a patient is insignificant. But some critics say a double standard is in effect: While clear protocols are in place to protect health care workers exposed to a patient's blood, no such protections exist for patients undergoing invasive procedures.

"Patients don't know when they've been exposed to blood - they're under anesthesia when this happens," said Janine Jagger, an epidemiologist who is director of the International Health Care Worker Safety Center at the University of Virginia Health System. "If there's no report of it in the record, then nothing is done about it.

"Patients never suspect this could happen to them," Dr. Jagger went on. "It's really swept under the carpet."

Health care workers, on the other hand, are required to report any exposure to a patient's blood so they and the patient can be tested and monitored, and they can take advantage of protective treatments like antiviral medications against H.I.V., Dr. Jagger said.

In recent years, meanwhile, Lawrence O. Gostin, a prominent public health law expert who works with the federal Centers for Disease Control and Prevention, has been urging health authorities to drop any restrictions that pertain to infected health care workers. Such rules, he says, lead to discrimination and discourage testing.

C.D.C. officials insist they are not planning to change the current policy, but they note that relatively few such infections from doctor or nurse to patient have ever been identified, even when retrospective studies have been done on patients treated by physicians later found to have the AIDS virus.

"In general, the risk for a health care worker transmitting hepatitis B, C or H.I.V. to a patient is very, very remote," said Dr. Elise M. Beltrami, a medical epidemiologist at the disease centers' division of health care quality promotion. "If we look at transmission in the health care setting, the biggest risk is to health care workers themselves."

In addition, she said, hepatitis B vaccinations of health care workers have made that virus less widespread.

But hepatitis C is more easily transmitted than H.I.V., and Dr. Jagger says monitoring is so spotty that it is impossible to know the number of health care worker-to-patient transmissions. In a sharply worded commentary last year in The American Journal of Infection Control, she and co-authors said there were "cavernous gaps" in the identification of worker-to-patient infections in the United States, and characterized the review of infected doctors' practices as "a capricious process that is all too vulnerable to local interests and conflicts of interest."

To some extent, the belief that such transmissions are extremely rare reduces the chance they will be identified; in the Long Island case, for example, the surgeon might never have been tested for hepatitis C if not for the chance encounter between two of his patients.

Public health officials are now investigating suspicious hepatitis infections among patients treated by a New York City anesthesiologist. So far they have asked some 4,500 patients to come in for testing, but have not publicly addressed whether the anesthesiologist has been tested.

Experts on both sides of the debate say adherence to strict infection-control and universal precautions - always wearing gloves when drawing blood, for example - are essential to protect both worker and patient. New technologies, like syringes with retractable needles, have drastically reduced needle-stick injuries in hospitals, though surgeons appear to be slower to adopt tools like blunt suture needles and scalpels with blade shields to prevent injuries.

Dr. Jagger and her colleagues have called for testing physicians for blood-borne pathogens before they start residencies in high-risk specialties, and for telling patients when they have been exposed to a health care worker's blood. Her group has also called for establishing a national reporting system for infection rates.

Health care officials urge patients not to postpone important medical procedures because they are worried about infections, since the risk is so remote. Patients may want to simply ask their surgeons in advance whether they are infected with hepatitis B or C or H.I.V., Dr. Jagger suggested.

But it is all too likely, she said, that the doctor has not been tested and will reply, in all truthfulness, "I don't know."