But the problems of rejection remain. In recent experiments involving transplanting pig organs into primates, many of the primates died from the massive amount of immunosuppressant drugs administered to prevent rejection.
Pigs have retroviruses mixed in their genes.
They are called porcine endogenous retroviruses or PERVs for short. These viruses don't harm pigs, but no one is sure what effect they may have when transferred into a human. The possibility of a disease jumping from one species to another is taken so seriously that anyone taking part in clinical trials will have to agree to conditions such as not having children and for any sexual partners to be monitored by doctors. The swine fever epidemic of 1920 killed more people than the First World War. Imutran is attempting to breed out many of the viruses and bacteria, but scientists have agreed that it is impossible to do this completely and are divided over what effect they may have.