Xenotransplantation (xeno- from the Greek meaning "foreign") is the transplantation of living cells, tissues or organs from one species to another such as from pigs to humans (see Meical grafting). Such cells, tissues or organs are called xenografts or xenotransplants. The term allotransplantation refers to a same-species transplant. Human xenotransplantation offers a potential treatment for end-stage organ failure, a significant health problem in parts of the industrialized world. It also raises many novel medical, legal and ethical issues. A continuing concern is that pigs have different lifespans than humans and their tissues age at a different rate. Disease transmission (xenozoonosis) and permanent alteration to the genetic code of animals are also a cause for concern.