Yellow fever is caused by the yellow fever virus, a 40- to 50-nm-wide enveloped RNA virus, the type species and namesake of the family Flaviviridae. It was the first illness shown to be transmissible by filtered human serum and transmitted by mosquitoes, by Walter Reed around 1900. Yellow fever begins after an incubation period of three to six days. Most cases only cause a mild infection with fever, headache, chills, back pain, fatigue, loss of appetite, muscle pain, nausea, and vomiting.