In 1900 the average life expectancy at birth in the United States was 47 years, and this figure increased to a record high of 77.2 years a century later. The gap between female and male life expectancy peaked in 1979 when women outlived men an average of 7.8 years. By 2001 the gap was down to 5.4 years. That year, women lived an average of 79.8 years and men an average of 74.4 years. White males averaged 75.0 years and black males 68.6 years; white females averaged 80.2 years and black females 75.5 years (Arias and Smith).
In 1900 more than half of the deaths involved young people, age fourteen and younger. By 2001, only 1.6 percent of the total reported deaths occur among young people. Heart disease and cancer are the leading causes of death in the early twenty-first century, together accounting for more than half of all deaths in the United States each year. In order, the top fifteen leading causes of death, comprising fully 83.4 percent of all U.S. deaths in 2001, were: heart disease, cancer, stroke, chronic lower respiratory diseases, accidents (unintentional injuries), diabetes, influenza and pneumonia, Alzheimer's disease, kidney disease, septicemia (infection from microorganisms), intentional self-harm (suicide), chronic liver disease and cirrhosis, assault (homicide), hypertension, and pneumonitis (inflammation of the lungs) due to solids and liquids.
In the past century the experience of death has changed from a time when the typical death was rapid and sudden, often caused by acute infectious diseases such as tuberculosis, typhoid fever, syphilis, diphtheria, streptococcal septicemia, and pneumonia, to a time when the typical death is a slow, progressive process. In 1900 microbial diseases, often striking rapidly, accounted for about 40 percent of all deaths; in the early 2000s accounted for only about 3 percent. In sum, in the past century U.S. society has evolved from one in which many children and young people died to a society in which death has become increasingly associated with older-aged people. The U.S. infant mortality rate reached a record low level in 2001: 6.8 deaths per 1,000 live births.
Observers of this phenomenon have proposed a theory of epidemiologic transition, a three-stage model that describes the decline in mortality levels and the accompanying changes in the causes of death that have been experienced in Western populations. The first stage, called the Age of Pestilence and Famine, is characterized by high death rates that vacillate in response to epidemics, famines, and war. Epidemics and famines often go hand in hand, because malnourished people are susceptible to infectious diseases. The second stage, the Age of Receding Pandemics, describes a time in which death rates decline as a result of the improved nutrition, sanitation, and medical advances that go along with socioeconomic development. The third stage, labeled the Age of Degenerative and (Hu)man-Made Diseases, describes the period in which death rates are low (life expectancy at birth exceeds seventy years) and the main causes of death are diseases related to the process of aging. The biggest challenge to this theory comes from the emergence of new diseases (such as AIDS/HIV, Legionnaires' disease, and Lyme disease) and reemergence of old infectious diseases (such as smallpox and malaria) in the latter part of the twentieth century. HIV/AIDS, for example, took the lives of between 1.9 million and 3.6 million people worldwide in 1999.
According to the Population Reference Bureau, life expectancy at birth for the world's population at the turn of the twenty-first century was 67 years—69 years for females, 65 years for males. In more developed countries life expectancy averaged 76 years—79 years for females, 72 years for males. In less-developed countries, life expectancy averaged 65 years—66 years for females, and 63 years for males.