These are thought to occur within the genome of every cell in every pig's body. The virus genomes have become part of the genome sequence of he pig and are passed on through the cells of the ovum and foetus. Thy are of no consequence to the pig or the pig farmer and are only of concern if the pig has been reared as a source of organs or tissues for human patients. The concern is that a retrovirus may pass from the transplanted organ or tissue into human cells and cause disease. The overriding fear is that this virus might then spread to other people causing an epidemic.