All animals carry inactive portions of retrovirus genes, eg env, as 0.1% of their DNA. These are termed endogenous viral elements. Infectious endogenous viruses are sometimes produced by continuous cell lines, eg from pigs and cats, and then replicate in human cell lines. If this occurred during xenotransplantation of humanised pig organs into man a new retrovirus could enter man. However 160 patients have received pig skin for burns,or pancreatic cells, or have been treated with extracorporal perfusion through pig spleen/kidney over 12 years without testing positive for porcine endogenous virus despite 23 patients having circulating porcine cells. However such xenotransplantation is likely to be banned especially once human organs can be grown in-vitro.