Diphtheria was a primary cause of death for most of human history, even though you may have never heard of it. It killed 8056 Americans in 1900, or 40.3 per 100,000.
Cancer was as prevalent in 1900s America as it is now. Before therapies, 12,769 Americans died from cancer and associated malignancies.
This category includes all 1900 US accidental deaths, which is interesting. Accidents killed 14,429 individuals, which appears modest given the US's 76.3 million population.
Nephritis is a simple condition that may require research. When kidney tissue gets inflamed, it stops filtering waste from the blood, killing 17,699 Americans in 1900.
Cerebrovascular Diseases killed 21,353 Americans in 1900 by restricting brain blood flow.
We're not talking about lovelust. 123 years ago, heart disease killed 27,427 Americans: the fourth most killer.
While gross, it's reasonable given the paucity of equipment to treat diarrhea and intestinal diseases, which killed 28,491 Americans in 1900.
In 1900s America, tuberculosis was terrible. Years of infection in the septum and lung destruction were typical of the condition. That could explain why 38,434 Americans died in 1900.