For sylvatic transmission, monkeys must be present at least part of the year. 1.3.3 Mosquito Vector Factors Not all mosquitoes can transmit the yellow fever virus. It is very important to know where Ae. This peri-domestic mosquito is an avid human biter and is usually the primary vector in epidemics. A person's immune status is the most important host factor.
People become immune after being vaccinated or after natural infection.
In populations where epidemics occurred in the past, or where vaccination has been erratic, the highest attack rates will occur in younger age groups. In populations that have not been exposed to either vaccine or natural virus, the attack rates and the severity of disease are the same for all ages, and males and females are equally affected. Social and cultural host factors also influence who becomes infected. For example, if transmission is sylvatic and men spend more time hunting or working in forested areas, then men may have higher attack rates. If the transmission pattern is urban, women and children spending more time near their houses may have higher attack rates.