In July and September, however, the daily dangerous gamble at the Endoscopy Center of Southern Nevada was lost. Six clinic patients were infected with hepatitis C, a disease that could cost them their lives.

That cluster of blood-borne disease cases, all painstakingly traced recently by public health investigators to procedures done at the clinic on July 25 and Sept. 21, forced health care officials Wednesday to alert 40,000 Nevadans they may have been exposed to HIV and hepatitis strains B and C.

It was the largest notification of its kind in U.S. history.

"When you realize that we average two cases of hepatitis C in this area per year -- sometimes we have none -- you realize just how rare what happened on those two days is," said Brian Labus, a senior epidemiologist at the Southern Nevada Health District.

The clinic was shut down Friday by city officials worried that even more disease could spread, but work on the case is far from over for Labus and staffers with the Centers for Disease Control and Prevention.

What happened on those two days in 2007 is a major focus. The reason: Your risk of contracting a blood-borne disease increases if an infected patient is treated before you on the same day.

"We are getting in touch by phone and letter as fast as we can with all 120 people who had procedures on those days," Labus said. "We want them to come in."

An investigation into the standard of care at the facility at 700 Shadow Lane by public health care professionals determined that patients who visited the center between March 2004 and Jan. 11, 2008, should get tested for the diseases as soon as possible.

The practice of reusing syringes to administer medications and the improper cleaning of colonoscopy equipment went on at least that long.

Labus said he and other researchers believe something far different must have happened on two days in 2007. Five people were infected on Sept. 21, one on July 25.

"You have to remember that only 20 to 30 percent of the people infected with hepatitis C ever show acute symptoms," he said. "These have shown up quickly. Does this mean that we have many more cases on the way? I hope not. What could have possibly caused so many acute cases on the same day?"

Labus said the 120 people called in will first receive the same kind of blood test that all 40,000 Nevadans who were exposed should get. If they test positive, genetic testing will follow to try and detect the specific strain of the virus.

"We want to identify the source. What we're doing is medical detective work. There's no other way to put it.

"Was the source a patient who somehow was more highly infectious? If so, why? How did it go to multiple patients so quickly on that day in September? Those are questions we want to answer."

All employees at the clinic have been tested, he said. None was positive.

What makes the detective work particularly challenging for public health investigators is the difficulty in determining where people were infected with hepatitis C.

Strains of the virus mutate quickly, making it more difficult to pin down the point of transmission, according to Dr. Scott Holmberg with the Centers for Disease Control and Prevention's Division of Viral Hepatitis.

Las Vegas attorney Ed Bernstein has filed lawsuits on behalf of two women who say they have contracted hepatitis C while undergoing colonoscopies at the Shadow Lane center, but Labus said their infections have not yet been connected to the center. One of the women, Deborah Hall-Hilty, said her procedure was on Oct. 20, 2006.

A woman contacted the Review-Journal Saturday and said she, too, had a colonoscopy on Oct. 20, 2006.

"Do you think I have hepatitis?" she asked, adding she will get tested Monday.

Holmberg explained last week that "unless we see a clear clustering ... it will be hard to impute transmission in the clinic setting."

Labus said the health district's interviewers are trained to deal with those who receive bad news.

"So far, I'd say we've received every kind of emotion from the people we've contacted," he said.

Some people break down in tears, while others' anger is off the charts, he said.

After a joint investigation by the Nevada State Board of Licensure and Certification and the health district, it was determined that syringes, not needles, and the use of vials of anesthesia medication on multiple patients were potential sources of infection.

A syringe would become contaminated by the backflow of blood when patients with a blood-borne disease were injected with medication, health officials said. That syringe, in turn, would be reused to withdraw medication from a different vial. That vial could become contaminated and result in infection.

While vials were inappropriately used at an affiliate of the Shadow Lane center, Desert Shadow Endoscopy Center at 4275 Burnham Ave., health officials found that syringes were not reused and no contamination resulted.

What angers Labus is that medical professionals who took oaths that they would not harm people are behind the outbreak that he says has captured the attention of public health officials around the globe.

"This didn't happen by accident," Labus said. "What happened here is something that every nursing and medical school teaches its students not to do. There is nothing they can possibly say that can justify what they've done."

Contact reporter Paul Harasim at pharasim @reviewjournal.com or (702) 387-2908.