The known and the unknown in yellow fever ecology and epidemiology.

Earlier epidemiological concepts are discussed briefly. Attention is focussed on several of the puzzling problems in the epidemiology of the disease as seen today. The recurring outbreaks of yellow fever in regions where it has been absent, as well as the absence of yellow fever in regions where the vectors exist are discussed. Features of vaccination are discussed, including the long persistence of humoral antibodies and long duration of effective protection. The possibility of changes in virulence of yellow fever strains is mentioned. In some of the modern outbreaks, virulence for human beings appears to be lower than virulence noted some centuries ago, and a hypothesis is advanced. The possible influence of virus-protecting antibodies acquired by infection by other flaviviruses is mentioned. With respect to the vectors of yellow fever, the possibility of differing susceptibility and of different behaviour of various strains is discussed. Transovarial transmission of the virus in the vector mosquito is mentioned as a possible mechanism for long persistence of the virus in nature. Infection of ticks and transovarial transmission of virus in ticks are also mentioned. The need for a diagnostic test to differentiate immunity produced by a naturally acquired infection from immunity induced by vaccination is stressed.