The ability to transplant bodily organs from one human to another is among the most significant advances of modern medicine. It may also lead to a most significant problem as organs become commodities -- and demand outstrips supply. PNS associate editor Anderson, author of "Evolution Isn't What It Used To Be" (W.H. Freeman), is a political scientist who writes widely on technology and global governance.

SAN FRANCISCO -- One of the great achievements of modern medicine is the ability to transplant organs -- move a liver, a kidney, even a heart from one human body to another.

In effect, this has created a completely new brand of medical treatment, adding to the traditional approaches of prevention, providing relief of symptoms, and curing disease. It has also created a completely new set of problems.

Organ transplantation was impossible until the 1980s. The procedure stems from a Swiss scientist's discovery of an unusual mold in Norwegian tundra. This mold is the source of "cyclosporin A," the drug that allows surgeons to suppress the immune system's rejection of an organ from another body.

The approach has saved countless lives, and created touching connections between people -- as when a family member donates a kidney to another, or the parents of someone killed accidentally arranges for their organs to be transplanted.

But there are complications, and they all boil down to one thing. Organs are now commodities. They can be bought and sold -- and they can also be stolen.

They are in demand -- a transplant can make the difference between life and death. And the demand exceeds the supply.

The gap between supply and demand is enormous and constantly growing as surgeons around the world acquire the necessary skills and more and more people learn that transplants are available.

According to the United Network for Organ Sharing, some 56,000 patients in the United States are waiting for an organ, and a new name is added to the list every 16 minutes. UNOS also points out that while the number of patients requesting transplants has skyrocketed over the past decade, the number of available donors has hardly changed.

Dr. David Rothman, a leading authority on the social dimensions of organ transplantation, notes that many religions and cultures believe in keeping the body intact after death -- but few have obstacles to being the recipient of something removed from such a body.

One result of this is a boom in "medical tourism," as people from countries where organs are not likely to be available (Israel, Saudi Arabia and Japan, for example) travel to countries where they are (such as Turkey, India, and China).

Different countries have evolved different laws on who owns a dead body. In the United States, your organs are available for donation only if you agree while you are alive, or if your next of kin consents after your death.

In some countries, however, a cadaver is considered property of the state, and every citizen is thus a potential donor. One such country is Belgium -- which has a surplus of organs and a thriving transplantation business, as the country does not allow organs to be exported. Foreigners who want a transplant must go to Belgium, stay in a Belgian hospital and pay Belgian doctors.

China's policy of using the bodies of executed criminals as a source of organs for transplant has proved controversial as some of the condemned are guilty of relatively minor crimes, and officials have been charged with arranging an execution for the convenience of an important recipient.

In India, it is reportedly fairly common for poor people to sell their own kidneys -- though it is technically illegal -- and there have been numerous rumors of children in developing countries being kidnapped and killed for their organs.

Whether or not these accounts are true, it is clear that the organs of the poor tend to find their way to the bodies of the rich.

Work is proceeding on substitutes for human to human transplants, involving artificial organs and animal donors -- but the commodity issue is not about to disappear. It is a major global problem, a symptom of injustice, and will not go away unless more people are willing to let others have their organs when they are no longer using them.