Abstract
The field of public health aims to improve the health of as many people as possible as rapidly as possible. Since 1900, the average life span in the United States has increased by more than 30 years; 25 years of this gain have been attributed to public health advances.1,2 Globally, life expectancy doubled during the 20th century,3 largely as a result of reductions in child mortality attributable to expanded immunization coverage, clean water, sanitation, and other child-survival programs.4

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