The extreme difficulty of obtaining sufficient organ donations means that many lives (and much quality of life) are lost each year due to the deficiency.

A promising possibility is xenotransplantation, in which animal, e.g. pig, organs are substituted for human ones. This is a surprisingly viable possibility, and would provide an array of advantages (other than perhaps to the pig.)

There are two significant problems. First, non-human organs tend to be rejected very quickly by human hosts. Second, pig cells in particular create a host of native viruses (even without infection) and there is significant concern that these might adapt to be new human pathogens if widely placed in human hosts.

Nonetheless significant progress is being made in addressing both challenges; for example this startup is apparently testing pig organ transplants into nonhuman primates.

When will the first pig (or other nonprimate) organ be successfully used in a human?

The organ should be a kidney, liver, heart, pancreas, or lung. We'll define "successful" as the organ functioning for 30 days after transplantation.