Yellow Fever

Yellow fever circulates in tropical parts of Africa and South America, cycling between mosquitoes and primates in forested areas before occasionally spilling into human populations near cities. Most infections are mild, but a minority progress to severe disease with high mortality. Because an effective vaccine exists, outbreaks often reflect gaps in vaccination coverage or interruptions in vector control rather than an absence of prevention tools. Virus Tracker highlights regional activity to complement vaccination campaign planning and travel guidance.

Yellow fever virus is transmitted by the bite of infected Aedes or Haemagogus mosquitoes. Two distinct cycles exist: a jungle cycle between mosquitoes and monkeys, and an urban cycle where mosquitoes spread the virus between people, which can fuel larger outbreaks in cities.

Initial symptoms include fever, chills, headache, muscle pain, and nausea. Most people recover after this phase, but roughly 15% enter a more severe second phase featuring jaundice (the 'yellow' in yellow fever), bleeding, and organ failure, which can be fatal.

A single dose of yellow fever vaccine provides lifelong protection for most people and is required for entry into some countries. Mosquito bite prevention and vector control add additional protection, particularly for unvaccinated travelers to endemic areas.

Surveillance combines case reporting, laboratory confirmation, and monitoring of vaccination coverage across at-risk regions. International Health Regulations require countries to report confirmed cases promptly given the outbreak potential in unvaccinated urban populations.

Related pathogens