Final answer:
The most severe short-term epidemic faced by America was caused by influenza, specifically the 1918 influenza pandemic known as the Spanish flu.
Step-by-step explanation:
The worst short-term epidemic experienced by America was caused by influenza. This refers to the 1918 influenza pandemic, commonly known as the Spanish flu, which was exceptionally deadly and quick-spreading. When comparing historical disease outbreaks, smallpox had a devastating long-term impact, especially among Amerindian populations following European contact, while the 1918 influenza had a severe short-term impact. Despite syphilis and tuberculosis being significant health concerns, they did not cause epidemics with such immediate and widespread mortality as the influenza pandemic did at the end of World War I. The influenza outbreak was one of the most lethal pandemics in history, with a death toll that far exceeded those of other diseases during similar time frames.