The Oregon Health Division said the case involves more than three dozen people who received tissue or organs from the infected man, who died of a hemorrhage inside his skull two years ago. Neither the man, tested for the disease in Oregon, nor the recipients, from 14 states and two other countries, were identified.

"This is probably a very rare case," Dr. Ann Thomas, a state medical epidemiologist who is investigating, said in Friday's editions of The Oregonian.

The organ donor, like all such donors, was tested for hepatitis C with a test that detects antibodies, virus fighters the body produces in response to an invasion by disease organisms. But the body takes six to eight weeks to produce the antibodies that specifically fight hepatitis C, Thomas said. During that time, a patient is unaware he or she has been infected.

That leaves a six- to eight-week window in which an infected donor can appear to be healthy. The disease leads to chronic liver disease in 70 percent of infected people.

"This was the classic window case," said Mike Seely, executive director of the Portland-based Pacific Northwest Transplant Bank, which arranged the original testing of the infected man.

Besides lacking hepatitis C symptoms, the man did not show any sign of being at significant risk for contracting the disease, such as being an intravenous drug user.

The case - first reported on by The Oregonian and the Milwaukee Journal Sentinel in Wisconsin - caught the attention of doctors in May, when a patient developed symptoms of an acute hepatitis C infection, according to a presentation last weekend by Dr. Barna D. Tugwell at the Interscience Conference on Antimicrobial Agents and Chemotherapy in San Diego.

Six weeks earlier, the patient had received a knee ligament from the infected man, who had died in late 2000, said Tugwell, an epidemic intelligence officer with the Oregon Health Division and lead investigator for this case.

In July, the donor's stored blood serum tested negative again both for the presence of hepatitis C antibodies and for the virus itself, but the direct test for the virus was positive for hepatitis C.

The Oregon Health Division immediately began notifying doctors of 40 patients who had received kidneys, lungs, liver, heart, vein, tendons, skin, bone and corneas.

So far, 18 of the 40 patients have concluded their tests for hepatitis C. Six were found to have become infected after their surgeries. Nine had no infection. Three had already been infected with hepatitis C before their transplants.

Tests on the remaining 22 patients are continuing.