Blood banks are so wary that they ask people who have had any organ or tissue transplant to wait 12 months before giving blood. After the mad cow scare of the 1990s, Americans who have lived in some European countries for only a year are banned as blood donors.
"The risks of transmission are approximately the same, and the tests that are applied to donated blood are, with respect to some diseases, actually more accurate than those used for organ donations," said Dr. James AuBuchon, professor of pathology at Dartmouth University's Hitchcock Medical Center in New Hampshire.
But, the guidelines for accepting organs is not as "codified" as for blood donations, according to AuBuchon, who serves on the board of the American Association of Blood Banks.
"The difference is the supply," he said. "Although we are frequently tight in our blood supply, there is the perception that we can find what we need despite the very strict health screening and testing requirements."
In organ transplants, the risk comes because some infections, like HIV/AIDS or rabies, can lie dormant. A donor organ can test negative, but disease can still develop up to a year later.
"The system does everything reasonable to prevent disease transmission," Dr. Vivian Tellis, head of the kidney transplant program at Montefiore Medical Center in New York, told ABCNews.com.
But critics say transplant hospitals have varying policies on informing patients about high-risk organs and — according to federal regulations — have no obligation to even tell the potential recipient.
"The issue is who should be making these policies – UNOS or the local transplant center?" asked Dr. Scott Johnson, surgical director of renal transplants at Beth Israel.
UNOS has recently written new regulations that require hospitals to obtain signed consent when using high-risk organs, and those policies are currently available for public comment.
Johnson estimates that about 5 percent of donated organs nationwide fall into the high-risk category. Most times, the risk of potential disease transmission is far outweighed by numbers of lives saved.
"It's about risk versus benefit," he said. "If you're sick in the ICU and you don't get an organ, you are going to die. You have to make a decision about what is available."
The "key component," according to Scott, is informed consent to ensure that organ recipients, their families or proxies understand the odds. But those risks are relative when a patient is clinging to life.
Once death has occurred, there is a finite amount of time before irreversible damage occurs in the organs, and the decision to go ahead with a transplant must be made quickly.
"It's up to the doctor to help the individual understand what the risk is, but sometimes even we don't even understand the risk," he said.
But Joan McGregor, a bioethicist from Arizona State University, says that organ donation networks are much more "closed" about their procedures than the national blood banks.
"The blood system went through some very public disasters with kids with hemophilia getting HIV and hepatitis from blood," she said, "My sense is that there should be more national regulations of the screening and the informed consent processes."
McGregor's concern is for the sickest of patients — those who may be in a "weakened state" waiting for a transplant — who may not be capable of making an informed decision about the risk of later developing a disease.