The Zika virus - suspected of causing brain damage in babies in Brazil - is likely to spread to all countries in the Americas except for Canada and Chile, the World Health Organisation has said.
Margaret Chan, the WHO director-general, told the organisation's executive board on Monday 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.
"The [WHO] regional directors and I are determined to change the way we respond to outbreaks and emergencies," Dr Chan said.
"The complexity of humanitarian emergencies underscores the need for transformational changes in our response capacity," she said.
Brazil's Health Ministry in November said that it suspected the mosquito-borne Zika virus was linked to a foetal deformation known as microcephaly, in which infants are born with smaller-than-usual brains. The WHO has described circumstantial evidence of the link as "suggestive and extremely worrisome".
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.
WHO advised pregnant women planning to travel to areas where Zika is circulating to consult a healthcare provider before travelling and on return.
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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.
There is currently no evidence of Zika being transmitted to babies through breast milk, the WHO said.
Source: Al Jazeera And Reuters