Yellow fever is a hemorrhagic fever – meaning it is caused by several distinct families of viruses. Most people who contract the virus have no symptoms, however the few that do experience these symptoms, feel them 3 days after infected. The symptoms include: headaches, muscle and body pains, fever, chills, nausea, vomiting, fatigue, and weakness. The yellow fever is spread by mosquitos, as well as sometimes ticks. It is originated in South America and Africa, therefore- the fever mainly spreads in hot, wet conditions such as in rainforests. The tissue that is invaded by the yellow fever is the respiratory system. The diagnosis is based mainly on blood tests that look for antibodies that one's immune system forms against the viral infection. The microbe that is found in yellow fever is transmitted from human to human through mosquito/tick bites. The virus multiplies in this insect vector. The bacteria is spherical (coccus), and the size ranges from 50 to 90nm. According to the CDC, the genus of yellow fever is Flavivirus Fibiscus. The genus is classified under a Group B Arbo–virus and is a member of the Toga virus family. It shares antigens with other members of the genus; (for example –the West Nile virus and Dengue.) The family of Flaviviridae is composed of over 70 other viruses along with yellow fever. Out of the multiple virus groups of the Flavivirus Species, yellow fever is unique. One way in which it is unique when compared to other viruses is that the feveris one of the very few diseases mosquitos are known to spread. Also, yellow fever is one of the few viruses where in most cases, the infected have no symptoms.