Officials from the World Health Organization said Thursday that the Zika virus was "spreading explosively" in the Americas and announced that they would convene an emergency meeting Monday to decide whether to declare a public health emergency.
"The level of alarm is extremely high," said Dr. Margaret Chan, the director general of the WHO, in a speech in Geneva.
Of particular concern, Chan said, are the cases of microcephaly, a rare condition in which infants are born with abnormally small heads that has been rising dramatically in Brazil as Zika spreads. Experts say it is too early to tell whether Zika is the cause of the condition, but there is growing evidence that the two are linked.
Health authorities in Brazil said Wednesday that reported cases of microcephaly had climbed to 4,180 since October, a 7 percent increase from the previous tally last week. Before the epidemic, Brazil recorded only about 150 cases of microcephaly a year.
Chan said she was "deeply concerned about this rapidly evolving situation." She also raised an alarm about the potential for further international spread of the virus, given how ubiquitous the mosquitoes that carry it are and how few people have developed immunity to it. The virus, which first surfaced in Uganda in the 1940s, had rarely been seen in the Americas.
"The level of concern is high, as is the level of uncertainty," she said. "Questions abound. We need to get some answers quickly."
Some experts had criticized Chan for not immediately convening a committee to advise on whether to declare Zika a public health emergency. On Wednesday in the journal JAMA, two experts called for an immediate meeting, saying the hesitation on the part of the WHO echoed the agency's slow reaction at the outset of the Ebola epidemic in 2014.
"The very process of convening the committee would catalyze international attention, funding, and research," they wrote.
On Thursday, one of the authors, Dr. Daniel Lucey, an expert on global viral outbreaks at Georgetown University School of Medicine, said of the announcement, "I'm very, very happy."
Chan said she would be asking the committee for advice on "the appropriate level of international concern" and for what measures the WHO should advise affected countries to take. She said she would also ask the committee to identify research priorities.
One worry, Chan noted, is that there is no vaccine against the virus, or a rapid diagnostic test to determine whether someone has been infected.
Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview this week that scientists at the National Institutes of Health were working on both.
"We are already on our way on the first steps to developing a vaccine," he said. "And we have started to work on a diagnostic to tell if someone's been infected."