Transplant surgeons work miracles. They take organs from one body and integrate them into another, granting the lucky recipient a longer, better life. Sadly, every year thousands of other people are less fortunate, dying while they wait for suitable organs to be found. The terrible constraint on organ transplantation is that every life extended depends on the death of someone young enough and healthy enough to have organs worth transplanting. Such donors are few. The waiting lists are long, and getting longer. Freedom from this constraint is the dream of every transplant surgeon. So far attempts to make artificial organs have been disappointing: Nature is hard to mimic. Hence the renewed interest in trying to use organs from animals. Doctors in India have just announced that they have successfully transplanted a heart from a pig into a person. Pressure to increase the number of such 'xenotransplants' (异种移植) seem to be growing. In Europe and America, herds of pigs are being specially bred and genetically engineered for organ donation. During 1996 at least two big reports on the subject—one in Europe and one in America were published. They agreed that xenotransplantation was permissible on ethical grounds, and cautiously recommended they be allowed. The ethics of xenotransplantation are relatively unworrying. People already kill pigs both for food and for sport killing them to save a human life seems, if anything, easier to justify. However, the science of xenotransplantation is much less straightforward. Import an organ from one animal to another and you may bring with it any number of infectious diseases. That much is well known. However, coping with this danger is not merely a matter of screening for obvious ills such as parasites. Many diseases that could harm humans may be both undetectable and harmless in their natural hosts. Diseases that have been dormant for years may suddenly become active if they find themselves in a new environment, such as a human recipient's body. After that, they may start to infect other people. Despite transplant surgeons' work, every year many people die because______.