[When some Flaviviruses are throwing our certainties]. / Quand certains flavivirus remettent en cause nos certitudes.

During the past two decades, a number of mosquito-borne flaviviruses, mainly of African origin, have invaded new geographical areas where they have never been active. This was the case for the Japanese encephalitis virus which reached the northeastern part of Australia (1995) and, above all, for the West Nile virus which, since 1999, entirely colonized the American continent. Then, the Usutu virus invaded a large part of the Western Europe (2001) while the Zika virus caused a large epidemic in an island of Micronesia (2007). Finally, in 2010, the Tembusu virus devastated many duck farms in China while the Bagaza virus, after having provoked human encephalitis in India, reached the southern part of Spain. In the affected areas, new pathogenic outcomes were observed in humans and animals while new vertebrate hosts and mosquito species were infected. Moreover, unusual ways of contamination were described. The origins of this unprecedented evolution remain to be clarified.