The development of vaccines has been one of the most important achievements in immunology and medicine to date. It was initiated almost two hundred years ago, by the famous trial of Jenner, who inoculated a small boy with cow pox, thereby achieving preventive immunity against the fatal small pox, a process which eventually led to the eradication of this disease. The major breakthrough, occurring just exactly one hundred years ago, was Pasteur’s development of rabies vaccine, a direct result of his pioneering systematic experiments on attenuation of microorganisms, which paved the way for the development of a whole series of vaccines based on viral attenuation. The presently existing vaccines, which consist of killed or live attenuated disease-causing microbial agents or their isolated components, have definitely diminished the incidence, morbidity and. mortality of a large number of infectious diseases, including major killers such as diphtheria and polio.