The Zika virus—linked to severe birth defects in thousands of babies in Brazil—is spreading “explosively” and could affect as many as four million people in the Americas, the World Health Organization (WHO) said on Thursday. Director-General Margaret Chan told members of WHO’s executive board that the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions and that she was convening experts to assess it. “Last year, the virus was detected in the Americas, where it is now spreading explosively.

As of today, cases have been reported in 23 countries and territories in the region,” Chan said, promising that the WHO would act fast. Last year the U.N. health agency was criticized for reacting too slowly to West Africa’s Ebola epidemic, which killed more than 10,000 people, and promised to cut its response time. “We are not going to wait for the science to tell us there is a link (with birth defects). We need to take actions now,” Chan said. FULL REPORT