Mosquitoes can transmit the yellow fever virus to humans after themselves having been infected by a viraemic person or monkey. Infection through blood transfusion, organ transplant, during pregnancy or breastfeeding is also possible. 

• About 1 in 10 people infected become ill. Symptoms can vary from a mild flu-like illness to a serious infection that can be fatal.
• After an incubation period of 2 to 9 days, the first symptoms appear, such a high fever, headaches and muscle pain. 
• Most patients recover after 3 or 4 days. However, around 15-25% of them fall ill again after a short recovery period, with severe symptoms, such as jaundice, bleeding and a state of shock. 
• Young children are more susceptible to severe forms of yellow fever (affecting the brain). Among people, who develop jaundice, the mortality rate is high, i.e. up to half of patients. 

• If the doctor suspects yellow fever, the diagnosis can be made by way of a blood test or lumbar puncture (in the case of severe illness). Depending on the stage of the disease, different tests can be used, such as a PCR test and serology.  
• There is no specific treatment for yellow fever, only symptomatic treatment.
• There is a very effective vaccine against yellow fever. In some countries, vaccination is mandatory for all travellers. Prevention is also done by avoiding mosquito bites.