Leading causes of death in the United States, 1900:

Tuberculosis - 11.3%
Pneumonia - 10.2%
Diarrheal diseases - 8.1%
Heart disease - 8.0%
Liver disease - 5.2%
Injuries - 5.1%
Stroke - 4.5%
Cancer - 3.7%
Bronchitis - 2.6%
Diphtheria - 2.3%

At the turn of the twentieth century, the lifespan expectancy for a newborn was about 49 years, although this differed greatly depending on race and sex. In 1900, a newborn white female had a life expectancy of just over 51 years, while a black male's was less than 34.

(Source: U.S. Centers for Disease Control and Prevention)