Characteristics

Yellow fever is normally a disease of nonhuman primates, particularly monkeys. Mosquitoes transmit the disease among them – but not mosquitoes that ordinarily bite humans. This form of the disease is jungle yellow fever or sylvan yellow fever; it is enzootic, meaning that transmission is from monkey to mosquito to monkey.

When the disease leaves the treetops, and mosquitoes (such as Aedes africanus and Aedes simpsoni in Africa, and species of Haemogogus in the Americas) begin transmission from nonhuman primate to mosquito to human, the disease is called endemic yellow fever. When the virus is carried by an infected human to populated areas, where transmission is from human to A. aegypti mosquito to human, the disease is termed epidemic yellow fever or urban yellow fever.

The habits of the female A. aegypti have much to do with shaping the characteristics of an epidemic. She is a domestic mosquito, living close to humans, depending on them for blood meals, and breeding in nearby loci of water. Her range is short, at most a few hundred yards, meaning that she requires a fairly dense human population. Because A. aegypti can survive only days without water (although her eggs can survive for years in dehydrated form) and requires water in which to breed, adequate rainfall is a prerequisite for epidemic yellow fever. Warm weather is another prerequisite: A. aegypti will not bite in temperatures under 62°F and hibernates in extended chilly weather.

The virus also has some distinctive requirements, especially for transmission – a process in which humans are best thought of as the site where the virus changes mosquitoes. This exchange can take place only during the first 3–6 days of infection of the yellow fever victim while the virus still remains in the blood (viremia); after the virus has entered the mosquito, it must incubate for another 9–18 days before the mosquito can infect another human. After this period of extrinsic incubation, however, the mosquito remains infective for the remainder of its life, which could be upward of 180 days, although generally the lifespan of the female A. aegypti is closer to a month or two.

In the Americas, epidemic yellow fever declined during the twentieth century, essentially because of efforts to eradicate A. aegypti in population centers. The last outbreak occurred in Trinidad in 1954. Nonetheless, the virus remains alive in the monkeys of Central and South America; consequently, some human cases are still reported among people who work or live close to the forests. Most cases occur in regions of Brazil, Ecuador, Venezuela, Colombia, and Peru that are drained by river networks contributing to the Orinoco, Magdalena, and Amazon systems. Earlier in the twentieth century, human cases were also reported fairly regularly in Central America, Bolivia, Argentina, and Paraguay.

In Africa, severe yellow fever epidemics still occur; a notable example, in Ethiopia in 1961, cost thousands of lives. Still more recently, a major outbreak in Nigeria claimed thousands more, and yet another was ongoing at the turn of the twenty-first century. A vast belt of endemic yellow fever stretches across Africa, but isolated cases are only irregulary reported.

One mystery surrounding yellow fever is its lack of incidence in Asia, despite the presence of Aedes mosquitoes. Some think that the mosquitoes themselves are resistant to infection. Others suspect that a population may support only so many arboviruses and that entrenched ones such as dengue and Japanese encephalitis may have forestalled the advance of yellow fever.

A yellow fever attack confers lifetime immunity on the host. Because the disease generally reserves severe symptoms for young adults and treats children more gently, whole populations in endemic or frequently visited areas can become more or less immune, with yellow fever just another childhood ailment. Under such circumstances, epidemics never occur unless groups of newcomers arrive, as was the case with immigrants, soldiers, and sailors reaching the Americas. It was this phenomenon that gave rise to yellow fever nicknames such as "strangers' fever," the "disease of acclimation," and "patriotic fever."

Many in the New World came to believe that blacks possessed a special ability to resist yellow fever. Most slaves reaching the Americas originated within the African endemic zone and would have acquired immunity before ever stepping aboard ship. Thus it is possible to explain blacks' refractoriness to yellow fever as acquired immunity.

On the other hand, genetic selection for yellow fever resistance as a result of prolonged exposure cannot be discounted, for many of the West African descendants of those first arrivals to the Americas lived for generations in areas untouched by yellow fever, yet, without any opportunity to acquire immunity in advance, suffered much less than whites when the disease finally did make an appearance. It has been suggested that related arboviruses or flaviviruses (dengue or Japanese encephalitis, for example) may confer some cross-protection against yellow fever, whereas others believe that certain strains of the illness may vary in mildness or severity, depending on the groups of individuals under attack. In this latter connection, it may be significant that Chinese in the New World were reputed to be almost as resistant to the illness as blacks, because although yellow fever has never invaded Asia, dengue and Japanese encephalitis are endemic to much of the region.