Yellow fever (also called yellow jack, black vomit, or sometimes American Plague) is an acute viral disease. It is still an important cause of hemorrhagic illness in several African and South American countries. Yellow fever is caused by an arbovirus of the family Flaviviridae, and is one of the smallest RNA viruses isolated. Human infection is begun after deposition of viral particles through the skin in infected arthropod saliva. Mosquitos are the primary vector in transmission of the disease from forest monkeys to humans and in person-to-person transmission. Yellow fever infections are frequently severe. A vaccine for yellow fever was developed which gives a 10-year or more immunity from the disease and effectively protects people traveling to the affected areas.