An organ donor infected four transplant patients with the AIDS virus in what a donor group says is the first such transmission in the U.S. in 13 years.

The transplants occurred in January at three Chicago hospitals. The patients infected with HIV and the virus for hepatitis C did not learn of their status until the last two weeks, according to medical officials.

Dr. Michael Millis, chief of the transplantation program at the University of Chicago Hospitals, said his staff was told of the problem on Nov. 1, and brought in the two patients who had transplants there for testing the next morning.

"It was very surprising and devastating for them, I'll be honest, just as it would be for any of us," Millis said.

Tests on the donor for HIV, hepatitis and other conditions came back negative, most likely because the donor had acquired the infections in the last three weeks before death. Personal details about the donor were not released by medical official officials, who cited privacy laws.

Based on the negative test results, doctors at Northwestern Memorial Hospital, Rush University Medical Center and the University of Chicago Medical Center went ahead with the transplants, based on the negative test results.

The right procedures were followed in testing the donor, said Alison Smith, vice president for operations at Gift of Hope.

Joel Newman, a spokesman for the United Network for Organ Sharing, said the last known example of HIV being transmitted from a donor to a recipient was in 1994.

Millis said he thinks the process can be improved but may never be completely failproof.

"The organ supply is extraordinarily safe, but this has demonstrated that it's not 100 percent safe and it is never going to be 100 percent safe, at least with technology we have today," Millis said.