Immunization has fundamentally altered the global infectious disease ecology. Powerful pathogens, once so prevalent that they toppled empires and laid waste to communities and cultures, are now only a distant memory. Smallpox today only exists in laboratories, and its danger now comes from its perceived threat if employed as a biological weapon. Polio persists in only a few corners of the world just 50 years after development of a vaccine. The indigenous transmission of measles has been drastically reduced throughout the Western Hemisphere. Congenital rubella has been eliminated from the United States and, in much of the world, entire organizations and institutions no longer exist as a result of successful disease elimination through immunization. In developed nations, orthopedic and rehabilitation hospitals have evolved into acute care children's hospitals or have gone out of business altogether, and charitable institutions (such as the March of Dimes) that were once solely dedicated to fighting infectious diseases like polio have now branched out into other efforts, such as the prevention of birth defects. For many today, deadly infectious diseases are a historic relic without a personal memory or current face. However, the emergence and spread of HIV and its resulting devastation in the latter part of the last century, and the recent emergence of a virulent new strain of avian influenza, coupled with the recognition that the deadly 1918 influenza pandemic was due to an avian influenza strain that adapted to human-to-human transmission, has made clear mankind's continued vulnerability to epidemic infectious disease.

Vaccines have contributed to our modern prosperity and have changed our perceptions of the world and ourselves. The success of immunization undergirds our twenty-first century notions, born of affluence, that health is a right, that technology will conquer all, and, mythically, that all risk can be eliminated. The tragic irony of this evolution is that these notions now threaten to reverse the gains achieved through widespread immunization. The rise in vaccine hesitancy, the high cost of technology, regulation, and legal defense, and the focus of public discourse on rare or unsubstantiated harms from vaccines without a concomitant understanding of the severity of the infectious diseases prevented by vaccines, all have the potential to contribute to a decline in the development, availability, and use of vaccines, and the potential recrudescence of diseases now controlled.

The challenge for the twenty-first century is to build on past success toward a future in which new vaccines are developed, vaccine safety and efficacy remain paramount, relevant old vaccines that are safe and effective remain in widespread use, and their benefits are accrued by all - and to do so without having to re-learn lessons of the past through the return of old infectious disease foes.

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