GENEVA – The World Health Organization declared a global emergency over the explosive spread of the Zika virus, which has been linked to birth defects in the Americas, calling it an “extraordinary event” that poses a public health threat to other parts of the world. The U.N. agency took the rare step despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil and following a 2013-14 outbreak in French Polynesia. Monday’s emergency meeting of independent experts was called in response to the spike in babies born with microcephaly in Brazil since the virus was first found there last year.