The World Health Organization rang a global alarm over the Zika virus on Thursday, saying the disease was “spreading explosively” in the Americas and that as many as four million people could be infected by the end of the year.
Dr. Margaret Chan, the director general of the W.H.O., said she was convening an emergency meeting on Monday to decide whether to declare a public health emergency. The move was a signal of how seriously the global health agency was treating the outbreak of the virus, which is transmitted by mosquitoes, after widespread criticism that it had allowed the last major global health crisis — Ebola — to fester for months without a coordinated, effective strategy.
“The level of alarm is extremely high,” Dr. Chan said in a speech in Geneva.
Of particular concern, Dr. Chan said, are the growing number of cases of microcephaly, a rare condition in which infants are born with abnormally small heads and damaged brains. Reports of babies born with microcephaly have 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 are some indications that the two are linked. The government of El Salvador has gone so far as to advise women to refrain from becoming pregnant until 2018.
But even as international health authorities sounded strong warnings, health officials in the United States sought to reassure Americans, saying that the vast majority of those who are exposed to the virus never get sick, and that the risk of an outbreak at home was very low, in part due to effective mosquito control.
“Right now I would say that the risk of Zika virus transmission over most of the United States is very low,” said Dr. William Reisen, editor of the Journal of Medical Entomology.
Dr. Reisen said the fact that American cities are less crowded and that the use of air conditioning and window screens is widespread means that it is very unlikely that the virus will spread in the United States as it has in less developed countries in South and Central America. He added that municipal services tended to be better in the United States than some rapidly expanding cities in the tropics.
Dr. Anne Schuchat, principal deputy director of the federal Centers for Disease Control and Prevention, said, “For the average American who is not traveling to this area, there nothing they need to worry about.”
Still, there was an urgency about the issue among American officials as well. Dr. Thomas R. Frieden, the head of the disease centers, briefed President Obama this week, and a team of experts from the C.D.C. is in Brazil working with authorities.
Global health authorities have a delicate task. They must impress upon the public and policy makers the seriousness of the outbreak to mobilize a proper response by governments and international health agencies, something that came too late during the Ebola epidemic. But the diseases are very different — Ebola was incredibly deadly and spread through contact with bodily fluids — and with inconclusive evidence that the Zika virus is the cause of the birth defects, health officials are cautious about drawing too dire a picture.
“ ‘Concerned’ is certainly the right language to be used,” Dr. Bruce Aylward, a W.H.O. assistant director general said in Geneva. “ ‘Alarmed’ would definitely not be the right language.”
About 31 cases of the virus have been reported in 11 states plus the District of Columbia since 2015, but all of those patients were infected in countries where the virus is widespread. Officials did not say whether any of them were pregnant. There were also 19 cases in Puerto Rico and one from the American Virgin Islands.
The health authorities in Brazil said on Wednesday that reported cases of microcephaly had climbed to 4,180 since October, a 7 percent increase from a week earlier. Before the epidemic, Brazil recorded only about 150 cases of microcephaly a year.
That has caused widespread alarm because researchers say the virus arrived in Brazil only recently, with the huge jump in microcephaly cases reported by doctors, hospitals and other medical officials following closely in its wake.
But proving that Zika is the cause has been elusive.
“It’s really important to understand the difference between associations and causations,” said Dr. Aylward said, noting that there are still many questions about whether the Zika virus and microcephaly are linked.
He said later: “Based on the information so far there is more and more concern that there may be a causal relationship but a lot of the work so far is to rule out other possible cause.”
The Brazilian health ministry said Wednesday that it had examined more than 700 reported cases of microcephaly and found Zika in only six of the infants — though what that means exactly is unclear. Infectious disease specialists caution that Brazil’s testing methods are outdated and may miss many Zika cases. They also say that in some cases, the mother may have had Zika, causing microcephaly in her baby, even if the virus is never detected in the infant.
The virus has spread to more than 20 countries and territories in Latin American and the Caribbean. Dr. 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.”
Asked whether the W.H.O. would advise people not to travel to Brazil for the Olympics, Dr. Aylward replied, “I would think that would be very, very unlikely when you look at areas affected and the scope of this.”
Some experts had criticized Dr. 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 W.H.O. 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 morning, one of the authors, Dr. Daniel Lucey, an expert on global viral outbreaks at Georgetown University School of Medicine, welcomed the announcement of the emergency meeting on Monday. “I’m very, very happy,” he said.
Dr. Chan said she would be asking the committee for advice on the “appropriate level of international concern” and for what measures the W.H.O. should advise affected countries to take. She said she would also ask the committee to identify research priorities.
One worry, Dr. 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.”
But Dr. Fauci said at a news conference that it was unlikely a vaccine would be widely available this year, or even in the next few years.