Zika virus outbreak

Declaring a threat of “alarming proportions,” the head of the World Health Organization warned Thursday that the Zika virus is “spreading explosively” across the Americas and could infect as many as 4 million people.

Dr. Margaret Chan told the agency’s executive board in Geneva that Zika has been linked in some places to an increase in the number of babies born with abnormally small heads and cases of Guillain-Barre syndrome, which can cause temporary paralysis.

Although there is no definitive proof that Zika is the cause, Chan said, “the possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions.”

Her agency plans to convene a meeting of independent experts Monday to decide whether the Zika outbreak should be declared an international health emergency.