At the turn of the 20th century, life expectancy in the United States was less than 50 years, but by century’s end it was approaching 80 years. A number of scientific discoveries and societal changes led to the dramatic increase in life expectancy that characterized the 20th century. These included developments in medial care and technology, scientific discoveries related to disease and improvements in sanitation and clean water for urban populations.
Winning the battle over many infectious diseases led to increased life expectancy. The Centers for Disease Control states that the primary causes of death in 1900 were pneumonia, tuberculosis, diarrhea, enteritis and diphtheria. A key component for battling these diseases and others was the scientific discoveries of bacteria and viruses, which led to the development of antibiotics, such as penicillin in the 1940s, and antiviral drugs to treat infectious diseases. For example, in 1900, the virus that caused yellow fever was the first identified in humans. Walter Reed discovered that mosquitoes transmitted the virus, and mosquito control led to its decline over much of the world.
Clean water and improved sanitation greatly reduced deaths due to infectious disease. Systems for filtering and chlorinating public water supplies is believed to have reduced urban mortality by half during the first part of the 20th century. Clean water is believed to have been the most important factor in the near disappearance of typhoid fever, a deadly and infectious waterborne disease, in the United States. A corollary to clean water were contemporaneous improvements in sewage disposal and treatment, which further contributed to decline in infectious disease.
Technology for diagnosing disease improved exponentially during the 20th century. Developments in serologic, or blood, testing that began in the early 1900s led to the ability to accurately diagnosis disease-causing bacteria. Subsequent technology to isolate and identify viruses led to an increasing understanding of how viruses work and spread. Technology continued to improve throughout the century, especially in the field of molecular genetics and disease. Computerization led to dramatic improvements in the ability to identify and treat diseases like cancer, and the 20th century witnessed major refinements in X-ray technology and the creation of imaging techniques like MRI scanning.
The combination of identifying disease-causing agents like viruses, major advances in medical technology and scientific discoveries in molecular biology all led to vastly improved treatment for disease throughout the 20th century. By midcentury, immunization programs for deadly childhood diseases such as polio, smallpox, diphtheria, measles and others saved numerous lives and contributed significantly to higher life expectancy. Immunizations were so successful that smallpox ceased to exist by century’s end, and others may disappear as well. Throughout the 20th century, the discovery and refinement of antimicrobial drugs like antiviral, antibiotic and antiparasitic drugs further reduced the mortality of infectious disease.