Yellow fever virus, a flavivirus, is transmitted to humans through the bite of infected mosquitoes. Illness ranges in severity from a self-limited febrile illness to severe hepatitis and hemorrhagic fever. Yellow fever disease is diagnosed based on symptoms, physical findings, laboratory testing, and the possibility of exposure to infected mosquitoes. There is no specific treatment for yellow fever; care is based on symptoms. Steps to prevent yellow fever include use of insect repellent, protective clothing, and vaccination. Yellow fever occurs in tropical regions of Africa and in parts of South America. Yellow fever is a very rare cause of illness in U.S. travelers. The last epidemic of yellow fever in North America occurred in New Orleans in 1905.