Agent Factors of Yellow Fever
Agent
The causative agent, Flavivirus Fibricus, formerly classified as a Group B Arbo–virus is a member of the Togavirus family. It shares group–specific antigens with other members of the genus (e.g. West Nile, Dengue). Under natural conditions, the virus is pantropic built after continued culture in tissues, as in chick embryo, it loses all its pathogenic properties but retains its antigenicity.
Reservoir of infection
In forest areas, the reservoir of infection is mainly monkeys and forest mosquitoes. In urban areas, the reservoir is man which has been substantiated through sub–clinical and clinical cases. Besides, another reservoir is the Aedes aegypti mosquito.
Period of communicability
Man: The blood of patients is infective, during which the virus multiplies in the insect vector. After infecting the mosquito, the virus has been shown to occur in adverse conditions (e.g. during extended dry seasons) in the absence of susceptible hosts.

Host Factors of Yellow Fever
Age and sex
People of all ages and both sexes are susceptible to yellow fever in the absence of immunity.
Occupation
A person whose occupation brings him/her in contact with forests (e.g. wood cutters, hunters), where yellow fever is endemic are exposed to the risk of infection.
Immunity
One attack of yellow fever provides life–long immunity, second attacks are unknown. Infants born of immune mothers have antibodies for up to six months of their lives.

Environmental Factors of Yellow Fever
Climate
A temperature of 24°C or over is required for the multiplication of the virus in the mosquito. It should be accompanied by relative humidity of over 60 per cent for the mosquitoes to live long enough to spread the disease.

Social factors of Yellow Fever
In Africa, urbanization has led to the extension of yellow fever. In addition, the expanding population has been encroaching upon areas that were previously sparsely populated, thereby bringing man within the radius of yellow fever infested areas. The increasing number of people who travel, and the greater speed with which they are transported from endemic areas to receptive ones, is also a cause for concern.