ABSTRACT
The art of deliberate immunization against infections has been practiced for centuries but the mechanisms of protective immunity were not fully appreciated until the advent of modern immunology. In spite of its various limitations, vaccination remains arguably the best answer to infectious diseases. Vaccinia virus is unique in that it retains infectivity after accommodating at least 25 Kbp of foreign DNA which is equivalent to about 20 average genes. Vaccination has been mainly responsible for the eradication of smallpox and for the control of yellow fever, poliomyelitis, and German measles in the human population, and of Newcastle disease, foot-and-mouth disease, and Marek's disease in domestic animals. The risk from viable wild-type vaccines is reduced by attenuation and eliminated by killing with heating or chemical treatment. The major tropical diseases, as listed by the World Health Organization special program on Tropical Disease Research, are malaria, schistosomiasis, leprosy, Chagas' disease, African trypanosomiasis, filariasis, and leishmaniasis.
