Four transplant recipients in Chicago, who received transplants at three local hospitals, contracted HIV from a single high-risk organ donor whose infection went undetected. The donor's tests for HIV, hepatitis and other conditions came back negative, but he or she had both hepatitis C and HIV.

This can be explained by the fact that the donor contracted the two diseases just before he died. A screening questionnaire determined that the organ donor had engaged in high-risk behavior, which probably means he was a gay man, but the hospital denied releasing any personal details about the donor or recipients.

"It's a risk-versus-benefit calculation," said Alison Smith, vice president for operations at Gift of Hope, as quoted by Chicago Tribune. "Every patient in need of an organ has a significant medical condition that in most circumstances limits life expectancy. The question becomes what degree of risk is appropriate in that situation."

The Centers for Disease Control and the Illinois Department of Public Health are currently investigating the case. The four critically ill recipients received the organ transplants back in January, but they learned of their diseases just two weeks ago. An incident like this hasn't happened since 1994, when a more sophisticated screening technique was put into place.

The four transplants took place at the Northwestern Memorial Hospital, Rush University Medical Center and the University of Chicago Medical Center, even though doctors knew the organs were coming from a high-risk donor. However, the doctors based their decision on the negative results of the blood tests and on the fact that the recipients were critically ill and would not have survived without a transplant.

"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," Dr. Michael Millis, chief of the transplantation program at the University of Chicago Hospitals, said.