Between 2005 and 2007, the Chikungunya virus (CHIKV) caused major epidemics that affected the islands of the Indian Ocean (IO) and the Indian subcontinent. Molecular analyses have shown that these epidemics were caused by a virus of African origin. A mutation in the gene encoding the viral envelope protein E1, selected during the Réunion epidemic and experimentally associated with increased infectivity in Aedes (Ae.) albopictus, likely facilitated virus transmission by this vector, explaining, at least in part, the scale of the epidemic. This mutation was independently selected in southern India and during other outbreaks where Ae. albopictus was the primary mosquito vector. For the first time, vertical transmission of CHIKV was demonstrated during the Réunion outbreak and was observed primarily perinatally. The recent emergence and epidemic transmission of CHIKV in Italy raises the question of CHIKV spreading to other European countries and underscores the need to strengthen entomological and human surveillance. (R.A.)