Historical events that changed the United States of America didn't always occur within its borders, such as one occasion that occurred in the spring of 1802.
The French conqueror Napoleon had sent a formidable army (led by his brother-in-law, General Charles Leclerc), to quell the rebellion of former slaves that was uprising in Haiti.
On April 19, Leclerc reported to Napoleon that the rainy season had arrived, and his troops were falling ill, at alarmingly high rates. By the end of the year, almost the whole French force, including Leclerc himself, were dead of mosquito-borne yellow fever.
When Napoleon came to the realisation that his re-conquest had failed, he abandoned all hope of forging an empire in the New World and decided to sell the Louisiana Territory to the United States.
And to think, if it wasn't for the mosquitoes, east coast USA might be a French colony today!