Many Americans remember Dr. Walter Reed as the man who led the research team that discovered that Aedes genus mosquitoes were the disease vector responsible for transmitting yellow fever virus. However, few people know about the man whom Dr. Reed himself credited with the discovery. This man's name was Carlo Finlay and this is his story.
From its introduction in the United States in 1699 through the early 20th Century, yellow fever killed thousands of Americans in cities across the country. An outbreak in Philadelphia in 1793 killed ten percent of the population, outbreaks in New Orleans over the 19th Century killed tens of thousands, and an outbreak affecting St. Louis, Memphis, Vicksburg, and New Orleans in 1878 killed more than twenty thousand. Most of these outbreaks originated in port cities, transmitted from ships arriving from the Caribbean countries, particularly Cuba.
Yellow fever was endemic in Cuba, but most Cubans had immunity to the virus, having been infected as children. Yellow fever is nowhere near as deadly to children as it is to adults, so very few Cubans died from the diseases. However, those not immune to the virus were at grave risk. During the Second War for Cuban Independence, an estimated 16,000 Spanish soldiers died from yellow fever, and after the Spanish-American War, an estimated 5,000 U.S. soldiers died from the disease.
Decades before the wars, in 1881, Cuban-Spanish physician Carlos Finlay made the hypothesis that Aedes genus mosquitoes were carriers of the diseases and transmitted the virus through bites. At the time, viruses were not known to exist, and diseases like yellow fever and malaria were thought to be caused by poor sanitation [malaria itself comes from the Latin words mal (bad) and aria (air)]. Yellow fever was actually the first virus to be discovered in history.
In 1900, a research team led by Walter Reed confirmed Aedes mosquitoes as the transmitters of yellow fever after extensive human experimentation and the deaths of one of the primary researchers, Jesse Lazear, and a nurse who volunteered to be infected, Clara Maass.
Although Carlo Finlay's original experiments were crude and discredited by many skeptics, his experiments formed the basis for Reed's later experiments and Reed sourced much of his information from papers written by Finlay. In addition, Finlay allowed Jesse Lazear to use his research and mosquito larvae. For this reason, Walter Reed credited Finlay for discovery the vector for yellow fever. For his work in the field, Carlos Finlay was awarded the Legion of Honour, France's highest decoration, and was nominated for the Nobel Prize in Physiology and Medicine seven times before his death in 1915.