The World Health Organization has warned up to 4 million people in the Americas could be infected with Zika virus by the end of this year. While the virus itself is usually not life-threatening, it appears to be linked to microcephaly, which causes babies to be born with abnormally small heads. At least 31 Zika cases have now been reported in Washington, D.C., and 11 states, including New York. All U.S. patients were infected abroad. Costa Rica has increased airport surveillance after confirming its first case, while officials in Colombia are fumigating homes. World Health Organization official Dr. Sylvain Aldighieri spoke in Geneva.

Dr. Sylvain Aldighieri: “If you start with a total number in the Americas of more than 2 million cases transmitted, reported of dengue per year, with a virus which is already circulating for years, you can come up with a figure between 3 and 4 million cases of Zika in the Americas.”

Scientists have linked rising temperatures from global warming to the increased incidence of mosquito-borne infections like Zika. WHO officials are slated to meet Monday to decide whether to declare a public health emergency.