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.
Age-adjusted death rates fell even 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 disease led to reduced mortality from them. Again developed countries felt the greatest benefit. In the 20th century, vacines became available for many diseases which caused deaths: Diphtheria, Pertussis, Tuberculosis, Tetanus, Yellow Fever, Polio, Measles, Hepatitis among others.