An attack of yellow fever confers on the host a lifetime of immunity against reinfection. Because the disease generally reserves its most severe symptoms for young adults and (as some illnesses) treats children more gently, whole populations in endemic areas or in areas frequently visited by yellow fever can become more or less quietly immune as children experience the illness as just one more in a train of childhood ailments. Under such circumstances an epidemic is never seen, unless or until large groups of newcomers suddenly arrive, such as was so often 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."

On the basis of the experience of both slave and free black populations in the hemisphere, however, many came to believe that blacks possessed a special (today some would say innate) ability to resist the disease. The question remains unresolved. Some have denied this ability, by pointing to urban epidemics in Ethiopia or Sudan and western Nigeria in which thousands of blacks lost their lives. These regions are on the edge of the endemic belt where many presumably would not have been exposed regularly to yellow fever, and thus would have included many with no acquired immunity. By contrast, most of the slaves reaching the Americas originated from deep within the endemic zone, and consequently would have acquired immunity to yellow fever before they ever stepped aboard a slaving ship. Thus it is possible to explain much of blacks' refractoriness to yellow fever on the basis of 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.

Fortunately today one need not suffer through the illness in order to ensure resistance to it. Rather immunization can be gained with a strain of living virus called 17D, and immunization projects have been in effect since the 1930s in many regions of endemic yellow fever.

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