Carlos Juan Finlay proposed in 1881 that mosquitoes caused yellow fever by means of a contagious pathogen. Walter Reed, who was leading a U.S. Army Yellow Fever Commission, discovered the yellow fever virus in 1900. He did so while investigating Finlay's theory in Havana City, Cuba. James Carroll, a doctor and one of Reed's partners, had an infected mosquito feed on himself. When he contracted an acute case of yellow fever, it gave proof to the theory that mosquitoes transmitted the disease.

The yellow fever virus does not cause the majority of those affected to feel ill. Nowadays, only some experience fevers, extreme headaches, fatigue, vomiting, and general pain. Out of these cases, only 15% develop a more advanced stage of yellow fever. This can cause jaundice, high fevers, bleeding (from the eyes, mouth and nose), seizures, and organ failure.

The pathogen is not contagious between humans and is transmitted by mosquitoes infected with the disease biting the victim. Mosquitoes, in turn, can acquire the disease by feeding on infected primates and then they can further spread the disease by biting other primates. In addition, people are infectious to mosquitoes right before the start of fever and for no longer than five days after the fever has started.

There are three different cycles by which yellow fever can be spread:

• The Jungle Cycle- when transmission is between monkeys and mosquitoes in the trees. These mosquitoes then bite humans in the area.
• The Savannah Cycle- when transmission is between mosquitoes and humans in jungle border areas.
• The Urban Cycle- when transmission is between humans and urban mosquitoes. This is typically caused by an infected person, who was in the savannah or jungle, being bitten.

The most effective means of prevention is a vaccination because it can prevent contraction of the disease for at least ten years, but more likely for the rest of the recipient's life. Usually within one week, the vaccination is working, at least for 95% of people. The other method of prevention occurs in areas without access to the vaccine: surveillance of the disease. If one case is confirmed, an immunization process can be put underway and the source can hopefully be eradicated by a team.

Because of the discovery of the yellow fever virus and how it spreads, scientists have learned more about how diseases spread through mosquitoes and how to stop them.

The research accompanying the virus furthered development for vaccines and modern medicine in Africa and other places where vaccines are need.

The World Health Organization now pushes for child immunization programs which will slowly decrease yellow fever cases as well as other fatal diseases that were once a major problem.

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