Yellow fever was the first human disease to have a licensed vaccine and has long been considered important to understanding how epidemics break out and should be combated. It was introduced to the Americas in the 17th century, and high death rates have resulted from successive outbreaks since then. Epidemics of yellow fever were associated with the slave trade, the U.S. gold rush and settlement of the Old West, the Haitian Revolution, and construction of the Panama Canal, to cite only a few examples.