The yellow fever virus is found in tropical and subtropical areas of Africa and South America. Yellow fever virus is an RNA virus that belongs to the genus Flavivirus. It is related to West Nile, St. Louis encephalitis, and Japanese encephalitis viruses.

Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes. For people who develop symptoms, the time from infection until illness is typically 3 to 6 days. Initial symptoms can include sudden onset of fever, chills, severe headache, back pain, general body aches, nausea, vomiting, fatigue (feeling tired), and weakness. Severe symptoms include high fever, yellow skin or eyes (jaundice), bleeding, shock, and organ failure