Yellow fever is transmitted by mosquitoes that live in tropical climates. In the early 1800s, it played a role in the Louisiana Purchase by weakening the French army.

Napoleon, leader at the time, planned to occupy the West and expand France’s influence around the world. French troops were stationed in Haiti when Haitian slaves rebelled successfully, defeating the French military. This was due to high rates of yellow fever among French troops, which weakened them and rendered them unable to fight back. In response to this, Napoleon offered up the whole Louisiana territory.

In the end, the purchase was settled as a treaty, allowing the land to be extremely inexpensive. Without yellow fever and the Haitian Slave Rebellion, The United States may have looked very different.