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. 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 subclinical and clinical cases. Besides, another reservoir is the Aedes aegypti mosquito. Host Factors Age and Sex People of all ages and both sexes are susceptible to yellow fever in the absence of immunity.
Infants born of immune mothers have antibodies for upto six months of their lives.
Environmental Factors Climate A temperature of 24 degrees Celsius 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 In Africa, urbanization has led to the extension of yellow fever. In addition, 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 areas, is also a cause for concern.