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Positive Externalities in Public Health Programs

One of the most remarkable changes in the standard of living in the last several centuries is that people are

living longer. Scientists believe that, thousands of years ago, human life expectancy ranged between 20 to 30

years. By 1900, average life expectancy in the United States was 47 years. By 2015, life expectancy was 79

years; due to COVID-19, life expectancy declined slightly to 77 years in 2020. Most of the gains in life

expectancy in the history of the human race happened in the twentieth century.

The rise in life expectancy seems to stem from three primary factors. First, systems for providing clean water

and disposing of human waste helped to prevent the transmission of many diseases. Second, changes in public
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