They produce high blood titres of virus which favours their chances of being picked up by a blood-sucking insect vector. Within that vector they can multiply again, which favours their chances of being transmitted from the insect vector to a new member of their definitive animal host. Crossing into non-normal hosts may produce disease. However, there are some notable exceptions to this generalisation, where human infection and disease can be spread to other humans via human-feeding insects or even directly person to person. Some vaccines but no specific treatments are available.
Human activity can bring man into contact with an otherwise silent cycle in nature, eg.
Creating vector promoting activities, such as dams and irrigation, may enhance human arbovirus disease potential. The virus is present in the insect saliva and the infection is lifelong in the insect. Yellow Fever, surveillance. By and large, there are 4 major clinical patterns of disease. Any one virus characteristically tends to produce mostly one pattern, but overlapping may occur.