Death rates in the 20th century from natural causes, including disease and malnutrition, plummeted in wealthier countries. In 1900 around 17 Americans per 1000 died in any given year. At the close of the century the number was around 9 per thousand.
Fertility rates and consequently live birth rates declined over the century, while age-adjusted death rates fell more dramatically. Children in 1999 were 10 times less likely to die than children in 1900.
For adults 24–65, death rates have been halved. The death rate for Americans aged 65 to 74 fell from nearly 7% per year to fewer than 2% per year.
The introduction of vaccines for several diseases led to reduced mortality from them. Again developed countries felt the greatest benefit. In the 20th century, vaccines became available for many diseases which caused deaths: diphtheria, pertussis, tuberculosis, tetanus, yellow fever, polio, measles, hepatitis among others.
However, war, genocide and Holocausts led to many millions of deaths throughout the century, and late in the century AIDS had already killed millions, particularly in Africa and south-east Asia. Cancer also killed millions via lifestyle and pollution generated by industrialization.