Brazil has deployed more than 220,000 soldiers across the country to warn people about the risks of the Zika virus.
The South American country is at the center of an outbreak of the Zika virus, which has been linked to a surge in babies being born with underdeveloped brains.
Brazil has 462 confirmed cases of microcephaly, and is investigating another 3,852 suspected cases.
Troops will hand out 4 million leaflets advising people about the risks of the virus, carried by mosquitoes.
However, critics have said the move would not helping reduce mosquito numbers or stop the spread of Zika.
The World Health Organization (WHO) has declared a global public health emergency over the possible connection between Zika and microcephaly, a condition marked by abnormally small heads in newborn babies that can result in developmental problems.
The link with Zika has not been confirmed, but the WHO and other public health bodies have said it is strongly suspected.
On February 12, the WHO said it expected that a link would be established within weeks between Zika, microcephaly and another neurological disorder, Guillain-Barre syndrome.
The microcephaly cases have been centered in north-east Brazil, but the Zika outbreak has affected people in more than 20 countries in the Americas.
Some governments have advised women to delay getting pregnant. Already-pregnant women have been advised not to travel to the countries affected.
Rio de Janeiro is to host the Olympic Games in August. A diving test event is to take place in the city next week, and organizers said on February 12 that some of the 270 athletes taking part had expressed their concern over the Zika virus.
Between three and four million people could be infected with Zika virus in the Americas in 2016, the World Health Organization (WHO) predicts.
Most will not develop symptoms, but the Zika virus, spread by mosquitoes, has been linked to brain defects in babies.
Meanwhile the US says it hopes to start vaccine trials in people by the end of the year.
According to WHO director general Dr. Margaret Chan, Zika had gone “from a mild threat to one of alarming proportions”.
Dr. Margaret Chan has set up a Zika “emergency team” after the “explosive” spread of the virus.
The emergency team will meet on February 1 to decide whether Zika should be treated as a global emergency.
The last time an international emergency was declared was for the Ebola outbreak in West Africa, which has killed more than 11,000 people.
Zika was first detected in Uganda in 1947, but has never caused an outbreak on this scale.
Brazil reported the first cases of Zika in South America in May 2015.
Most cases result in no symptoms and it is hard to test for, but WHO officials said between 500,000 and 1.5 million people had been infected in Brazil.
The virus has since spread to more than 20 countries in the region.
At the same time there has been a steep rise in levels of microcephaly – babies born with abnormally small heads – and the rare nervous system disorder Guillain-Barre syndrome.
The link between the virus and these disorders has not been confirmed, but Dr. Margaret Chan said it was “strongly suspected” and was “deeply alarming”.
She also warned the situation could yet deteriorate as “this year’s El Nino weather patterns are expected to increase mosquito populations greatly in many areas”.
One hospital in Recife, north-east Brazil, had gone from dealing with an average of five cases of microcephaly a year to 300 in the past six months.
Earlier, doctors writing in the Journal of the American Medical Association said Zika had “explosive pandemic potential” and said the WHO’s failure to act swiftly on Ebola probably cost thousands of lives.
In a statement to the executive board meeting of the WHO, Dr. Margaret Chan said: “The level of concern is high, as is the level of uncertainty.
“Questions abound – we need to get some answers quickly.
“For all these reasons, I have decided to convene an Emergency Committee.
“I am asking the Committee for advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere.”
Officials from the US National Institute of Health (NIH) said they had two potential Zika vaccines in development.
One that is based on an experimental West Nile vaccine could be repurposed for Zika and enter clinical trials by the end of 2016, Dr. Anthony Fauci from NIH said.
Dr. Anthony Fauci said talks were already taking place with pharmaceutical companies, but a vaccine would not be widely available for several years.
Meanwhile Dr. Anne Schuchat, from the Centers for Disease Control (CDC) confirmed there had been 31 cases of Zika in the US – all linked to travel to the affected areas.
At a news conference, White House spokesman Josh Earnest said the US response to the virus so far had been “consistent with the kind of threat that could be out there”.
“At this point, here in the United States, the risk of a disease spread by mosquitoes is quite low, the January temperatures in North America are quite inhospitable to the mosquito populations.”
“But, obviously that’s going to change,” he added.
Dr. Carissa Etienne, the regional-director for the WHO Pan American Health Organization, said the link between the abnormalities and Zika had not been confirmed.
American specialists have urged the World Health Organization (WHO) to take urgent action over the Zika virus, which they say has “explosive pandemic potential”.
Writing in the Journal of the American Medical Association, the scientists called on the WHO to heed lessons from the Ebola outbreak and convene an emergency committee of disease experts.
They said a vaccine might be ready for testing in two years but it could be a decade before it is publicly available.
Zika, linked to birth defects as microcephaly, has caused panic in Brazil.
Thousands of people have been infected there and it has spread to some 20 countries.
Brazilian President Dilma Roussef has urged Latin America to unite in combating the virus.
Dilma Rousseff told a summit in Ecuador that sharing knowledge about the disease was the only way that it would be beaten. A meeting of regional health ministers has been called for next week.
In the JAMA article, Daniel R. Lucey and Lawrence O. Gostin say the WHO’s failure to act early in the recent Ebola crisis probably cost thousands of lives.
They warn that a similar catastrophe could unfold if swift action is not taken over the Zika virus.
“An Emergency Committee should be convened urgently to advise the Director-General about the conditions necessary to declare a Public Health Emergency of International Concern,” Daniel R. Lucey and Lawrence O. Gostin wrote.
They added: “The very process of convening the committee would catalyze international attention, funding, and research.”
White House spokesman Josh Earnest said on January 27 the US government intended to make a more concerted effort to communicate with Americans about the risks associated with the virus.
There is no cure for the Zika virus and the hunt is on for a vaccine, led by scientists at the University of Texas Medical Branch.
The researchers have visited Brazil to carry out research and collect samples and are now analyzing them in a suite of high-security laboratories in Galveston, Texas.
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