WHO spokesman Christian Lindmeier says the "big task" is to try to establish a link between the virus and microcephaly, which involves abnormally small heads in newborns and can affect brain development.
He said the UN agency plans a special session Thursday on the virus during a Geneva meeting of its executive board. He said the virus has been associated with close to 4,000 microcephaly cases in Brazil, and El Salvador, Panama, Colombia and Cape Verde also have "large outbreaks."