The complex epidemiological process of arboviral and arenaviral diseases, their dependence on a multitudine of etiologic agents, the risk of severe manifestations, absence of a specific treatment and difficulties in preparing the vaccines, place these diseases among those of high concern at national and international level. The multidisciplinary investigations carried out in Moldavia by M. Duca, Eugenia Duca, Gr. Teodorovici, Alla Vâţă, St. Dumitriu, V. Luca, D. Buiuc, A. Ivan and others have elucidated numerous aspects regarding the epidemiology and prevention of arboviral diseases. The increased morbidity by meningitis and meningoencephalitis recorded in Romania the interval summer--early fall 1996 was attributed based on serologic diagnosis, to West Nile virus. The etiological investigations were done both in our country (Cantacuzino Institute, Institute of Virology, Laboratory of the Ministry of National Defence) and abroad (Pasteur Institute at Paris, Colindale Institute London, CDC, Atlanta, USA). For the Romanian population the risk for a periodical re-emergence of some arbovirus diseases is due to favorable local conditions with the relationships with geographical areas were these diseases have and endemic-epidemic evolution are added.