In areas of Africa with persistent circulation of YF virus, natural immunity accumulates with age consequently, infants and children are at greatest risk for disease. Urban YF occurs periodically in Africa and sporadically in the Americas. Most YF disease in humans is due to sylvatic or intermediate transmission cycles. YF occurs in sub-Saharan Africa and tropical South America, where it is endemic and intermittently epidemic (see Table 5-22 and Table 5-23 for lists of countries with risk of YF virus transmission).

One case of perinatal transmission of wild-type YF virus from a woman who developed symptoms of YF 3 days prior to delivery has been documented the infant subsequently tested positive for YF viral RNA and died of fulminant YF on the 12th day of life. Because of the high level of viremia, bloodborne transmission theoretically can occur via transfusion or needlesticks. Humans infected with YF virus experience the highest levels of viremia shortly before onset of fever and for the first 3–5 days of illness, during which time they can transmit the virus to mosquitoes. The urban cycle involves transmission of virus between humans and peridomestic mosquitoes, primarily Ae. YF virus can be transmitted from monkeys to humans or from human to human via these mosquitoes. In Africa, an intermediate (savannah) cycle involves transmission of YF virus from tree hole–breeding Aedes spp. Virus is transmitted from monkeys to humans via mosquitoes when occupational or recreational activities encroach into the jungle. The sylvatic (jungle) cycle involves transmission of virus between nonhuman primates and mosquito species found in forest canopies. YF virus has 3 transmission cycles: sylvatic (jungle), intermediate (savannah), and urban. Nonhuman primates and humans are the main reservoirs of the virus, and anthroponotic (human-to-vector-to-human) transmission occurs. Vectorborne transmission of YF virus occurs via the bite of an infected mosquito, primarily Aedes or Haemagogus spp. Yellow fever (YF) virus is a single-stranded RNA virus that belongs to the genus Flavivirus. State health department or contact CDC’s Arboviral Diseases Branch (97 Infectious Agent

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