The incubation period is three to 12 days, typically five to seven days. Viraemia may persist up to 12 days (typically four to five days) (Wichmann and Jelinek 2004). The importance of this information becomes clear when considering unwell travellers returning from the tropics. The viraemic phase of the illness means that mosquitoes taking a blood meal on an infected host during this period become infected disease vectors.