Global average life expectancy has more than doubled in the past century, rising from about 32 years in 1900 to 71 years by 2021. No single breakthrough explains this. Instead, a combination of cleaner water, childhood vaccines, better nutrition, and modern medicine each chipped away at the causes of early death, compounding over decades into one of humanity’s most dramatic transformations.
The Biggest Factor: Fewer Children Dying
The most important thing to understand about life expectancy is that it’s an average. When a large number of children die before age five, the average plummets, even if many adults live to 60 or 70. In 1900, infectious diseases like diarrhea, measles, and pneumonia killed children at staggering rates. That pulled the overall number down far more than adult deaths did.
The decline in child mortality is the single largest reason life expectancy has climbed so dramatically. In 1990 alone, 12.8 million children under five died worldwide. By 2023, that number had fallen to 4.8 million, a 59% drop. Neonatal deaths (in the first 28 days of life) fell from 5.2 million to 2.3 million over the same period. Each of those saved lives adds decades to the average, which is why the early-20th-century gains in life expectancy were so steep.
Clean Water and Sanitation
Before cities built modern water treatment and sewage systems, waterborne diseases like cholera, typhoid fever, and dysentery were routine killers. Contaminated water still causes an estimated 2.9 million cholera cases and 95,000 deaths globally each year, and unsafe water and poor sanitation account for roughly 88% of diarrhea-related deaths in developing countries. In the early 1900s, those numbers were far worse everywhere.
Water chlorination, which became widespread in the United States and Europe in the first two decades of the 20th century, was arguably the single most cost-effective public health intervention in history. Combined with enclosed sewage systems that kept human waste out of drinking water, these infrastructure changes eliminated entire categories of disease from daily life in industrialized countries within a generation.
Vaccines and Antibiotics
Mass vaccination programs transformed childhood survival. Before vaccines, diseases like diphtheria, pertussis (whooping cough), and polio were constant threats. In the Netherlands, diphtheria accounted for about 1.4% of the total childhood mortality burden, pertussis for 3.8%, and similar patterns held for polio, mumps, and rubella. Once mass vaccination campaigns began, those contributions dropped to near zero almost immediately. Researchers estimate that vaccination programs in the Netherlands alone averted roughly 9,000 childhood deaths and 148,000 years of life lost among children born before 1992.
Antibiotics added another layer of protection. Penicillin became widely available in the 1940s, turning previously fatal bacterial infections like pneumonia, tuberculosis, and infected wounds into treatable conditions. For the first time, a cut that became infected or a case of strep throat no longer carried a real risk of death.
Better Nutrition and Food Fortification
Chronic malnutrition weakens the immune system and stunts development, making people far more vulnerable to infections. As agriculture industrialized and food distribution improved throughout the 20th century, outright starvation became less common in much of the world. But it wasn’t just about calories. Specific nutrient deficiencies caused diseases that are now almost forgotten.
Rickets (from lack of vitamin D), pellagra (from lack of niacin), beriberi (from lack of thiamine), and goiter (from lack of iodine) were all widespread in the early 1900s. Food fortification, adding vitamins and minerals to staples like milk, bread, and salt, virtually eradicated these conditions in the United States. Johns Hopkins University has called food fortification one of the most effective methods ever developed to prevent nutritional deficiencies. These weren’t exotic interventions. They were simple changes to everyday foods that quietly saved millions of lives.
Conquering Heart Disease
Once infectious diseases were largely controlled, heart disease and stroke became the leading causes of death in wealthy countries. But even these have been beaten back dramatically. Age-adjusted heart disease death rates in the United States fell 56% between 1950 and 1996. Stroke death rates dropped 70% over the same period. Since 1978, cardiovascular death rates across the industrialized world have fallen to roughly one-third of their 1960s peak.
About half of this decline came from medical treatments: drugs to lower blood pressure and cholesterol, procedures to open blocked arteries, and better emergency care for heart attacks. The other half came from people changing their behavior. Reductions in smoking contributed about 12% of the decline. Lower cholesterol levels (partly from dietary changes, partly from medication) contributed 24%. Better blood pressure control added another 20%. Physical activity improvements chipped in 5%. It was a genuine partnership between medicine and prevention.
Why Progress Has Stalled in Some Places
Despite a century of gains, life expectancy is not rising everywhere. The United States actually saw small declines during the 2010s, even before COVID-19. The pandemic accelerated that trend, but it wasn’t the only cause. Drug overdose deaths (driven largely by the opioid epidemic), rising suicide rates, liver disease linked to alcohol use, obesity-related conditions, traffic accidents, and homicides have all contributed.
Researchers at the University of North Carolina have described this as “a story of an unhealthy society,” where overlapping crises eroded gains that took decades to build. The U.S. now has a lower life expectancy than most other wealthy nations, despite spending far more on healthcare per person. This gap highlights that life expectancy depends not just on medical technology but on how well a society addresses addiction, mental health, diet, and access to care.
What Drove Each Era of Gains
The causes of rising life expectancy shifted over the century. Roughly speaking, the first half was dominated by public health infrastructure and infectious disease control: clean water, sanitation, vaccines, and antibiotics. These interventions saved the most lives among the young, which is why life expectancy climbed so rapidly in that period.
The second half was more about chronic disease. As populations aged and infectious deaths declined, heart disease, cancer, and stroke became the primary targets. Advances in treatment, along with public health campaigns against smoking and poor diet, extended life at the other end, keeping older adults alive longer. The average 65-year-old today can expect to live significantly longer than a 65-year-old in 1950, which would not have been true if the gains were only about childhood survival.
Looking ahead, the focus is shifting again. Precision health approaches that use data to identify disease risk early, mobile health platforms that make prevention more accessible, and better management of chronic conditions before they become severe are expected to drive the next wave of gains. The biggest opportunities likely lie not in new miracle drugs but in getting existing knowledge to more people, more quickly, and more equitably.