An estimated 350 million people are chronic carriers of the virus. Infection is less common in Western Europe and North America, where less than 1 percent are chronic carriers. Immunization is the only effective method to prevent the disease in infants. Yellow Fever Yellow fever is an acute infectious disease that is caused by the yellow fever virus. Humans may acquire the infection through the bite of an infected mosquito.
Yellow fever is now a serious public health threat again.
It has caused large epidemics in Africa and the Americas. The WHO estimates that there are some 200,000 cases of the disease and 30,000 deaths each year. Immunization is the most effective method to prevent the disease. In urban yellow fever, large epidemics can occur when migrants introduce the virus into areas with high human population density. Aedes aegypti mosquitoes carry the virus from person to person.