Forecasts of human life expectancy are an important component of public policy because they influence the funding for, and solvency of, age-entitlement programs. In the United States the Social Security Administration (SSA) recently decided to raise their estimates of how long Americans are going to live in the 21st century. However, current trends in childhood and adult obesity in the U.S. and other low mortality populations and the global re-emergence of communicable diseases, pose serious threats to the health and longevity of present and future generations. Furthermore, death rates and life expectancy at older ages in the U.S. have remained relatively constant for the past twenty years. In this paper we present empirical evidence demonstrating the existence of these trends and discuss their possible affect on life expectancy, as well as the public health measures required to mitigate them. We believe there is sufficient evidence to support the conclusion that unless broad scale public health measures are enacted to address the obesity epidemic and rise of communicable diseases, human life expectancy could decline in the 21st century.