Scientists are concerned about the spread of disease, through, as was mentioned before, limited genetic diversity. In sum, diversity is absolutely necessary to have healthy livestock. The creation of genetic uniformity is actually already happening through selected breeding, for example some dog breeds which are so overbred have developed hip dysplasy. Uniformity will cause a much greater potential for epidemic disease wipeouts. Also the use of organs produced by animals, pigs for example, could cause some serious problems.
Scientists know that pigs can easily get retroviruses.
Therefore, they are worried about implanting pig organs in human bodies. Even clones could initiate plagues of retroviruses that would not appear until years after the initial infection and that humans would have no resistance or drugs to fight. But if getting a new heart from a pig could save a patient who suffers from a lethal heart disease, he or she would probably be thankful. Either the patient takes the risk of getting a virus or he or she might die immediately because of an unhealthy organ, like a heart or a kidney. In addition to clearly dangerous consequences to the human species, some people object to cloning on purely personal, religious and ethical grounds.