The history of yellow fever, and how it was determined that the disease was transmitted by mosquitoes. Included: the work of Carlos Finlay, the Cuban physician who found the link to the insects; how Finlay's theory influenced Jesse Lazear and James Carroll, scientists who were part of Walter Reed's team after Reed was sent by the U.S. to Havana to find the cause of the disease when American troops were sent to Cuba following the Spanish-American War.