This issue has been raised between scientists back from 1997, when Dr. Robin Weiss and others found that swine virus PERV can infect human cell lines. It is not that hard to imagine such possibility exists since viruses jump around between different hosts occasionally. However, what is more is the paper points out that when we transplant pig tissues for human, such as heart, kidney, etc, this has a potential to become a real issue. By transplanting the pig tissues into human body, we are creating an environment that human cells and pig cells are close to each other and that might increases the risk of viruses of one host to another.
Since this blog is pro-evolution and is not anti-virus, anti-vaccine, anti-organ transplant, or anti anything, I would suggest to think in a more interesting trajectory: Xenotransplantation not only changes the human body but also the microbes on this body. In a way almost like using artificial methods to change human physicality in order to encourage evolution for other beings that symbiosis with us. If xenotransplantation does gives viruses a direction to evolve then can we assume that eventually the force of changes (of viruses) will come back to other human bodies that are not modified in the form of viruses?