It's possible there could be viruses we don't know about that could be released into the human population. All pigs contain several copies of Pervs. Viruses related to PERV are associated with cancers of the blood. We already know that viruses have jumped the species barrier as a result of xenotransplants. In the US, a man who received a baboon liver in an experimental transplant became infected with baboon-specific CMV virus.
All viruses mutate.
This is not science fiction or a Stephen King novel. This is very real and very possible. We cannot screen for viruses we don't know exist, we can't eliminate the proviruses, we can't guarantee pig organs without Pervs and we can't rely on the physicians, scientists and vested interest groups to act responsibly. Others consider that to be a conservative estimate. The vested interest groups who hype xenotransplantation portray it as making life-saving organs available to people.