Transmission
Yellow fever spreads via the bite of Aedes and Haemagogus mosquitoes that are active during daytime hours. Repellent, long sleeves and accommodation with screens reduce exposure, but vaccination is the only reliable defence.
Geography
Endemic across tropical sub-Saharan Africa (the “yellow fever belt”) and the Amazon basin in South America. Most countries inside these zones require an International Certificate of Vaccination (ICVP) at the border — including some non-endemic countries if you have transited through a risk country.
Immunity
A single dose now confers lifelong protection for nearly everyone aged 9 months and over, in line with WHO guidance. The certificate becomes valid 10 days after vaccination.