The Zika virus is transmitted mainly by the Aedes aegypti mosquito, which also carries the dengue and chikungunya viruses. It is unusual in that it has evolved to breed in urban areas in small pockets of water such as in discarded tyres and potplant trays. It feeds exclusively on human blood, and during the day, making it a formidable disease threat across huge swathes of tropical and subtropical regions. To a lesser extent, at least so far, Zika can also be spread by Aedes albopictus, whose distribution reaches far into temperate zones.