Historically, yellow fever has dominated arboviral diseases in South America and the Caribbean and sylvatic cases still occur every year (more than 200 in 1979). The dengue, by the very large number of cases occurring in the area during successive epidemics--the most important being the last one in 1977-1978--is a serious problem for public health, an epidemic of hemorrhagic dengue being a constant threat. Encephalitides are the third syndrome of arboviral etiology observed in the South American subcontinent. Some which preferably attack horses have been known for a long time whereas the one due to the Rocio virus of an even graver prognosis, was recently discovered. These diseases present a large range of epidemiological forms. Dengue is an example of an urban illness transmitted from man to man through the mosquito, Aedes aegypti. But most of the viruses circulate between vectors and wild vertebrates in sylvatic cycle. For some viruses, both mechanisms can be involved. It is the case for Oropouche virus and yellow fever even if at present the urban form is the only recorded for the latter.