Carlos Juan Finlay was born on December 3, 1833 in Puerto Principe (now Camagüey), Cuba. He was the son of Scottish-born Dr. Edward (Eduardo) Finlay and French-born Elisa (Isabel) de Barrés. His father was a physician who had fought alongside Simón Bolívar, and his family owned a coffee plantation in Alquízar. He attended school in France in 1844, but was forced to return to Cuba after two years because he contracted cholera. After recovering, he returned to Europe in 1848, but stayed in England for another two years due to political turmoil, and after arriving in France to continue his education, he contracted typhoid fever and again returned to Cuba.
The University of Havana would not recognize his European academic credits, so he enrolled at Jefferson Medical College in Philadelphia, Pennsylvania, which did not require prerequisites. Finlay met John Kearsley Mitchell, a proponent of the germ theory of disease, and his son Silas Weir Mitchell, who supervised his studies. He graduated from Jefferson Medical College in 1855. He then returned to Havana and set up an ophthalmology practice in 1857, and then studied in Paris from 1860–61.
He was the first to theorize, in 1881, that a mosquito was a carrier, now known as a disease vector, of the organism causing yellow fever: a mosquito that bites a victim of the disease could subsequently bite and thereby infect a healthy person. He presented this theory at the 1881 International Sanitary Conference, where it was well received. A year later, Finlay identified a mosquito of the genus Aedes as the organism transmitting yellow fever. His theory was followed by the recommendation to control the mosquito population as a way to control the spread of the disease. His hypothesis and exhaustive proofs were confirmed later by the Walter Reed Commission of 1900. Finlay went on to become the chief health officer of Cuba from 1902 to 1909. Although Reed received much of the credit in history books for “beating” yellow fever, Reed himself credited Finlay with the discovery of the yellow fever vector, and thus how it might be controlled. Reed often cited Finlay’s papers in his own articles and gave him credit for the discovery in his personal correspondence.
This discovery helped William C. Gorgas reduce the incidence and prevalence of mosquito-borne diseases in Panama during the American campaign, from 1903 onwards, to construct the Panama Canal. Prior to this, about 10% of the workforce had died each year from malaria and yellow fever.
Finlay was a member of Havana’s Royal Academy of Medical, Physical and Natural Sciences. He was fluent in French, German, Spanish, and English and could read Latin. His interests were widespread and he wrote articles on subjects as varied as leprosy, cholera, gravity, and plant diseases. His main interest, however, was yellow fever, and he was the author of 40 articles on this disease. His theory that an intermediary host was responsible for the spread of the disease was treated with ridicule for years. A humane man, he often took on patients who could not afford medical care. As a result of his work, Finlay was nominated seven times for the Nobel Prize in Physiology or Medicine, although he was never awarded the prize. He received the National Order of the Legion of Honour of France in 1908.
Finlay died from a stroke, caused by severe brain seizures, at his house in Havana on August 20, 1915.