When the United States declared its independence in 1776, the threats to life looked almost nothing like those Americans face today. Diseases that once swept through towns have largely disappeared, replaced by chronic conditions that come with modern life and longer lifespans.

According to experts, the shift in what kills Americans reveals not only revolutionary progress in medicine but also a new era of challenges in health and lifestyle.

Kenneth J. Perry, M.D., an emergency physician in Charleston, South Carolina, told Fox News Digital, “The amount of changes that have happened over the past 250 years are immeasurable when it comes to life expectancy and disease.”

He noted that the average life expectancy in the late 18th century was roughly 30 years, compared with close to 80 years today.

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In 1776, no national mortality records existed. But historians generally agree that infections and epidemics were responsible for most deaths. Lacking vaccines, antibiotics, and an understanding of germs, Americans were highly vulnerable to diseases that are now preventable or treatable.

Poor sanitation, unsafe drinking water, and the absence of refrigeration made foodborne and waterborne illnesses rampant. Hospitals were rare and surgery was performed without sterile tools or anesthesia.

Among the leading causes of death were smallpox, tuberculosis, pneumonia, dysentery, malaria, yellow fever, and typhoid fever. Complications from childbirth also claimed many women’s lives, while infant mortality was extremely high—about 10% to 30% of infants did not survive their first year.

Outbreaks such as the yellow fever epidemic in Philadelphia in 1793 left cities in crisis as carts collected the dead. Infections from wounds and childbirth were often fatal, since antiseptics and antibiotics had not yet been discovered.

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By contrast, the first national mortality statistics, published in 1900 by the U.S. Census Bureau, reveal how much conditions had changed by the turn of the 20th century. Pneumonia, tuberculosis, and diarrheal diseases together accounted for nearly one-third of all deaths, and about 30% of those who died were children under age five.

At the same time, Americans were beginning to benefit from a series of public health revolutions. Cities built sewage systems and introduced clean water supplies. Scientists uncovered germ theory in the late 1800s, transforming medicine with sterile surgical practice and infection control.

The discovery of vaccines and the use of penicillin in the 1940s further reshaped life expectancy. Once-fatal diseases like smallpox, polio, diphtheria, and measles were brought under control, while smallpox became the first human disease to be eradicated worldwide in 1980.

Advances in obstetric care, such as blood transfusions, antibiotics, Cesarean sections, and neonatal intensive care, drastically improved maternal and infant survival rates. Heart care also changed with the development of CPR, defibrillators, bypass surgery, stents, and statins.

These breakthroughs helped shift the leading causes of death away from infections and toward chronic conditions such as heart disease, stroke, and cancer. The National Cancer Institute found that prevention and early detection have been key in saving lives, with screening and lifestyle changes accounting for about 80% of cancer deaths averted over the past 45 years.

Today, chronic diseases dominate because most people live long enough to develop them. Yet physicians warn this progress brings new concerns tied to modern living habits.

Dr. Omer Awan of the University of Maryland School of Medicine explained to Fox News Digital, “The transformation of deaths in the last 250 years, largely from infectious diseases to currently chronic debilitating diseases, represents both success and new challenges Americans will have to face.”

He added that sedentary lifestyles and diets high in fat, salt, and ultraprocessed foods fuel these chronic epidemics, leading to obesity and related illnesses.

Still, Awan said the pattern of progress can continue. “Just as vaccines and antibiotics prolonged life centuries ago, so can lifestyle changes, exercise and new therapies that target obesity—like GLP-1 drugs and medications that promote better metabolic health.”

What killed Americans in 1776 was largely out of their hands. Today, the greatest threats to health stem not from infection, but from behavior and longevity itself. The change tells a story of innovation, survival, and the ongoing evolution of how a nation manages to live—and live longer.