In developing countries, infectious diseases still cause 30% to 50% of all deaths. Effective chemotherapeutic agents simply do not exist for many of the diseases that plague these regions, and many of the agents that do exist are far too costly for much of the population to afford. Vaccines thus have become the most important tool for fighting infectious diseases in those parts of the world.

The situation is very different in developed countries, where infectious diseases account for only 4% to 8% of all deaths. This is not to say, however, that vaccines are not important in those parts of the world. The low rate of infectious diseases in industrialized nations is in fact largely the result of the widespread use of vaccination (Figure 5.1). In addition to the well-known example of the smallpox vaccine, which has succeeded in eradicating the disease completely, other vaccines have brought dramatic decreases in the incidence of numerous grave diseases. For example, at the beginning of the twentieth century, diphtheria (caused by the bacterium Corynebacterium diphtheriae) infected about 3000 children yearly out of every million in developed countries. Because diphtheria targets young children in particular, this incidence corresponds to several percent of children of the susceptible age, and nearly one tenth of the infected children died. Now, thanks to a mass immunization program, diphtheria incidence in the United States is less than 0.2 per million, a decrease of more than a thousandfold.