The agency said the emergency is warranted because of how fast the mosquito-borne virus is spreading and its suspected link to an alarming spike in babies born with abnormally small heads -- a condition called microcephaly -- in Brazil and French Polynesia.
Reports of a serious neurological condition, called Guillame-Barre Syndrome, that can lead to paralysis, have also risen in areas where the virus has been reported. Health officials have specifically seen clusters of this in El Salvador, Brazil and French Polynesia, according to WHO's Dr. Bruce Aylward.
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The conditions have not yet been conclusively linked to the virus.
"The experts agreed that a causal relationship between Zika infection during pregnancy and microcephaly is strongly suspected, though not yet scientifically proven," Dr. Margaret Chan, the WHO director-general said.
But the large area potentially affected by the virus, the lack of vaccines and reliable diagnostic tests, and lack of population immunity in the affected countries contributed to the need for the declaration, according to the WHO. Read more on edition.cnn.