ABSTRACT
As described above, genetic changes ocrur naturally on a continual basis. Humans have exploited these changes in the shape of attenuated vaccines over many years. Many micro-organisms will become naturally attenuated during serial passage through hosts. The classic example of this occurrence is the attenuation of Myxoma virus following release into the wild rabbit population of Australia. Whereas initially the virus was highly lethal, non-lethal mutants which resulted in highly infectious rabbits surviving for long periods of time, hence increasing the chances of virus transmission, were naturally selected (Fenner and Ratcliffe 1965).
