Dr. Margaret Chan, director-general of the World Health Organization, said at a special meeting Thursday in Geneva that the Zika virus — which has been linked to birth defects and neurological problems — was “spreading explosively.”

Chan said although there was no definitive proof that the Zika virus was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, “the level of alarm is extremely high.” She also noted a possible relationship between Zika infection and Guillain-Barre syndrome, which can cause temporary paralysis.

“The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions,” Chan said.