Humans and monkeys are the principle reservoirs for the virus. The most common types of mosquito that transmit the yellow fever virus are the Aedes aegypti or Haemagogus spp (which only occurs in South America).

There are three main transmission cycles. Sporadic cases resulting from sylvatic (jungle) transmission are seen in South America and Africa. The intermediate cycle of transmission occurs in the moist savannah zones of Africa only, when semi-domestic mosquitoes infect both animals and humans and may cause small epidemics in rural villages. Urban transmission can occur where the virus is introduced into urban areas and the domestic Aedes aegypti mosquito is present. This can lead to large epidemics if the virus is introduced into unvaccinated populations.

Yellow fever is rare in travellers, but since 1996 there have been six fatal cases in European and US travellers. All the fatal cases were in unvaccinated travellers.

The yellow fever vaccine is very effective and safe, although there have been a few reports of rare adverse events associated with the vaccine. Yellow fever vaccine is an entry requirement for some countries, but it is advised that travellers to yellow fever endemic areas should be vaccinated even if there is no specific requirement. More information about yellow fever vaccine for travellers is available from the National Travel Health Network and Centre  (pdf 370 kb).