Like thousands of other patients do every year, John Leto walked into a Glen Burnie cardiology clinic last October for a routine cardiac stress test.

But what happened over the next two months wasn't so routine: The 79-year-old retired ironworker developed a hepatitis C infection that ultimately took his life on Christmas Day.

Leto's death - one of just six hepatitis C-related fatalities officially recorded in Maryland since 1999 - is drawing new attention to an unusual medical mystery under investigation by the Maryland Department of Health and Mental Hygiene.

Officials have traced Leto's infection to a single vial of technetium-99m, a radioactive isotope injected into the bloodstream during stress tests and other routine diagnostic procedures.

Investigators won't say how many people have been infected, or where they live. However, the company whose Timonium pharmacy prepared the suspect isotope said the state has identified at least 12 people in the Baltimore area. Leto, who lived in Brooklyn Park, is the only fatality.

The case has baffled physicians and nuclear medicine experts, who say they can recall no other instance in which a common radioactive isotope has become contaminated with hepatitis.

"It's very unusual and not expected at all," says Fadia Shaya of the University of Maryland School of Pharmacy, who chairs the state's advisory council on hepatitis C. "This kind of thing should not happen."

Hepatitis C, a liver disease caused by a virus of the same name, is typically transmitted through infected blood or semen. The disease kills as many as 10,000 people in the U.S. each year. The majority of new infections are contracted through illicit drug use.

The vial of technetium-99m that officials suspect as the cause of the infection here was prepared at a so-called nuclear pharmacy in Timonium. Operated by Cardinal Health of Dublin, Ohio, the pharmacy specializes in preparing radioactive "tracers" for a variety of diagnostic tests. The company has temporarily closed the business until investigators have pinpointed the source of the virus.

John Hammond, a spokesman for the Department of Health and Mental Hygiene, said the hepatitis C outbreak does not pose a public health risk and was confined to the small number of people injected with serum from the tainted vial.

Maryland, he said, recorded 26 cases of hepatitis C in 2004, including those in the recent outbreak. The state reported nine cases the previous year and 14 cases in 2002.

One of the central mysteries is how the radioactive isotope could have become contaminated, especially since the preparation of technetium-99m is considered to be a straightforward process.

"How in the world did this happen?" says Nicki Hilliard, a nuclear pharmacist at the University of Arkansas for Medical Sciences. "It's bizarre."

Hilliard said that the isotope is typically produced in a pinkie-sized container known as a radionucleotide generator. To create a dose, technicians pass standard saline solution through the generator, a process known as "milking." As a result, the generators themselves are sometimes known as "cows."

Technicians capture the saline in a small vial and stir in a powdered chemical tracer. The tracer binds to the isotope and ferries it through the patient's bloodstream to the specific organ or tissue that needs to be examined.

Dorothy Leto, the victim's widow, said her husband received the isotope during a cardiac stress test at Arundel Heart Associates in Glen Burnie. Before the test, she said, he was in good health, and the procedure was considered routine.

As part of the test, doctors typically inject the isotope into the patient's bloodstream and ask the patient to walk on a treadmill for a short period to increase his pulse rate. Then they examine his heart using a radiation-sensitive camera to track blood flow and spot potential blockages.

Dr. Paul Young-Hyman, a cardiologist at the Glen Burnie clinic, said the suspect isotope arrived at the clinic in October in eight individually prepared syringes, each in a lead-lined container. All eight patients who received the isotope on Oct. 15 have since tested positive for hepatitis C, he said.

Young-Hyman said that when word started filtering back that several patients who had stress tests that day had been diagnosed with hepatitis, the clinic notified county health officials, who ultimately contacted the state.

The cardiologist said all the clinic's employees have tested negative for the virus, and health officials have examined the clinic's laboratory and found it contamination free. As a precautionary measure, however, stress tests at the clinic are now being performed with a different radioactive tracer.

"I'm just hoping that people who need the study don't hold back on getting it," says Young-Hyman. "This case really does fall into the category of bizarre."

Hepatitis C has an average incubation period of six to eight weeks, and Leto was fine until mid-November, when he lost his appetite and started to feel nauseous, family members said.

His wife initially chalked it up to stomach flu, from which she was just recovering. But when he didn't improve, he went to see the doctor. A few days later, tests came back positive for hepatitis C.

Around this time, state health officials began notifying hospitals and physicians to watch for signs of the disease, including fatigue, jaundice, abdominal pain and nausea.

On Thanksgiving, a few days after his 80th birthday, Leto and his wife went to the home of Leto's daughter, Delma Macey, in Severna Park. When he walked through the door, Macey was shocked - he looked gaunt and his normally tan skin was now a sickly orange from jaundice. "I went into the room and just fell apart," Macey recalled.

As his condition deteriorated in the following weeks he was admitted to North Arundel Hospital twice before finally being taken to Harbor Hospital on December 23.

He died at 2:45 p.m. Christmas Day.