Zika has not yet been reported in the continental United States, although a woman who fell ill with Zika in Brazil later gave birth to a brain-damaged baby in Hawaii.
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Brazil has reported 3,893 suspected cases of microcephaly, the WHO said last Friday, over 30 times more than had been reported in any year since 2010.
The disease's rapid spread, to 21 countries and territories of the region since May 2015, is due to a lack of immunity among the population and the prevalence of the Aedes aegypti mosquito that carries the virus, the WHO said in a statement.
Evidence about other transmission routes is limited.
"Zika has been isolated in human semen, and one case of possible person-to-person sexual transmission has been described. However, more evidence is needed to confirm whether sexual contact is a means of Zika transmission," it said.
It advised pregnant women planning to travel to areas where Zika is circulating to consult a healthcare provider before travelling and on return.
WHO Director-General Margaret Chan told the WHO executive board that she had asked Carissa Etienne, head of the WHO in the Americas, to brief the board later this week on the WHO's response to the outbreak.
"An increased occurrence of neurological symptoms, noted in some countries coincident with arrival of the virus, adds to the concern."