However, using animals to grow organs for human transplant remains controversial. Determining which swine genes cause rejections could take many years and many generations of cloned pigs. Other researchers worry that diseases found in animal genes may be passed to patients through the organs. Scientists fear that these viruses may infect humans and even cause an epidemic. The scientific journal, Nature, reports that test mice receiving pig tissue transplants did become infected with a swine virus but survived.
Animal viruses can also change unpredictably when they jump the species barrier.
Today and Tomorrow Some animal organs are already used to keep humans alive for short periods of time. Pig livers, for example, are transplanted into patients whose own livers have failed. These livers are not permanent organs. They simply serve to keep the patient alive until a human organ becomes available. Patients are given anti-rejection medication so that their bodies will not immediately reject the animal organs.