is likely that some local infection of host cells would occur. The subsequent events necessary for generation of pathogenic, transmissible viruses increasingly are unlikely, but on some unknown, arbitrary scale. Although the probability of inadvertent creation of a new epidemic generally is judged to be extremely small (particularly given the long history of intimate association between humans and pigs), it cannot be ignored altogether. Current FDA policy is to permit xenotransplantation trials to proceed, but to require close monitoring of recipients, and (insofar as possible) of their contacts (DHHS, 2001). Attempts also are being made to identify specific proviruses responsible for production of infectious virus and then to selectively breed them out of lines of animals to be used as transplant donors (Herring et al., 2001).

TABLE 3.4 Theoretical scale of risks associated with PERV transmission from xenotransplants.

Event	Cumulative Probability
Expression of infectious virus
Localized infection of host cells
Spreading infection in the host
Persistent viremia
Disease (e.g., lymphoma, “AIDS”)
Transmission to close contacts
Spreading, epidemic transmission