  The early discussion of ethical issues raised by chimeras occurred under the heading of xenotransplantation, the use of non-human animals as a source of cells, tissues, or organs for transplantation into humans (Institute of Medicine 1996). The moral argument in favor of xenotransplantation was one of beneficence: the shortage of human organs available for transplant and the inadequacy of alternatives meant that if non-human animals could serve as a source of organs, hundreds of thousands of human lives could be saved or improved each year (ibid., 14). The main ethical problems included the health risks for the transplant recipient (e.g., a substantial risk of hyperacute rejection and graft-versus-host disease), traditional animal ethics issues, concerns about informed consent (complicated by empirical uncertainties and the possibility of legally mandated life-long health surveillance), fair allocation of health care resources, and the public health issue that xenotransplantation would allow viruses to jump the species barrier into humans.