Four people were infected with HIV and hepatitis C in January 2007 after receiving organs from a person who turned out to be infected with both viruses. The cases, reported in November, once again raised concerns in the public's mind about these medical procedures.

The risk of contracting HIV or hepatitis C infection differs depending on whether a person is receiving a blood transfusion or a transplanted organ, so let's consider them separately.

Blood: All blood donated in the United States undergoes a many-layered process to reduce the risk of passing an infection from the donor to the recipient. This includes interviewing and examining the donor to find out about any risk factors for or symptoms of infectious disease, and then, according to the American Red Cross, performing laboratory tests on blood samples for viruses and other infectious agents, including tests for:

• Syphilis
• HIV
• Hepatitis B
• Hepatitis C
• West Nile virus

Testing for HIV and HCV is actually done twice: once to look for antibodies to these viruses, and again to look for nucleic acid (DNA or RNA), the virus's genetic material. That is because each of these infections is associated with what is known as a "window period"--a short period of time when a recently infected person might test negative for antibodies to these viruses. Using both types of tests improves the chances of finding infection during this early period, although, very rarely, an infectious agent still goes undetected. Currently, the risk of becoming infected with HIV or hepatitis C is about 1 in 2 million for each unit of donated blood. This is extremely low.

Organs: Blood from potential tissue or organ donors is also tested for antibodies to HIV and hepatitis C before decisions about whether to transplant tissue or organs into waiting recipients are made. People who know a potential donor are also asked to furnish information about the donor's medical history. Transplanting organs from HIV-positive donors is prohibited. Organs from donors with hepatitis C may be transplanted in certain situations, such as those in which the recipient also has hepatitis C.

The antibody tests are quite good at detecting most cases of HIV and hepatitis C infection in the donors, but there is a small risk that an infection might go undetected during the window period (see above). Unfortunately, although antibody testing can be done very rapidly, performing a follow-up nucleic acid test is sometimes not possible because it takes longer to obtain results, during which time the available donor organ might deteriorate too much, the patient needing the transplant might die of disease complications, or both.

From 1985 through 2007, 8 organ or tissue recipients appear to have been infected with HIV from HIV antibody-negative donors. However, given that more than 400,000 organs have been transplanted in the last 20 years, the risk of infection remains low.