The World Health Organisation's declaration that the Zika outbreak is a global health emergency is only the fourth time the United Nations health body has used its powers to require international collaboration to solve the problem.
The body is enacting Public Health Emergency of International Concern measures which sound a worldwide alarm to member states who are now expected to send aid and expertise to struggling countries.
The state of emergency could also lead to trade and travel routes being shut while speeding up research into possible treatments and vaccines.
The Brazilian President's chief of staff says it will take researchers between three and five years to develop a vaccine against the Zika virus.
Jacques Wagner said Brazilian researchers were working with researchers in the United States. "If we are really lucky, it could be three years. But it could be between three and five years."
WHO officials said Zika had moved "from a mild threat to one of alarming proportions". Up to 4 million people could be infected this year, they predict.
The virus is being linked to microcephaly (an unusually small head) in newborns.
WHO director Dr Margaret Chan said the disease was placing a "heavy burden on families and women".
"After a review of the evidence the [emergency] committee advised that the causes of microcephaly constitute an extraordinary event, and a public health threat to other parts of the world," said Chan.
"In their view a co-ordinated international response is needed to minimise the threat in affected countries."
The WHO was widely criticised for its slow response to the 2014 Ebola crisis in West Africa and has been keen to show it can respond quickly to a crisis.
Professor Peter Piot, from the London School of Hygiene and Tropical Medicine, said: "By any means this is a public health emergency with the sheer numbers of people who are coming down with a flu-like syndrome, but particularly the complications. Some people develop neurological symptoms, sometimes weeks after the infection, and also the impact on the foetus. There is a very strong association between microcephaly, small brain, small skull in infants and Zika.
"Declaring a public health emergency will, first of all, draw attention that this is something that will spread further. This has implications say for example travellers who will know there is an alert, if they go to Brazil or El Salvador for Carnival."
International Olympic Committee president Thomas Bach says he's confident there will be good conditions for athletes and spectators at the Rio de Janeiro Games in August. That will be in Brazil's winter when cooler temperatures can cut down mosquito populations.
Zika is now in 25 countries, most in southern and central America, and is being linked to nearly 4000 babies being born with microcephaly.
Pregnant women have been warned not to visit countries with the disease and couples have been advised to delay pregnancy by up to six months if either has experienced unexplained rashes or fevers after trips to infected regions.
The WHO declaration came just hours after Brazilian President Dilma Rousseff authorised health officials to enter private properties by force if necessary to eradicate mosquitoes. Officials are looking for breeding grounds for mosquitoes that can carry the virus, which has spread rapidly over the Americas and particularly in Brazil.
Zika was first identified in 1947 in a Ugandan forest, but until last year it wasn't believed to cause any serious effects.
'An ethical nightmare'
The Zika virus outbreak in Latin America could be a bigger threat to global health than the Ebola epidemic that killed more than 11,000 people in Africa.
Jeremy Farrar, head of the Wellcome Trust, said: "Most virus carriers are symptomless. It is a silent infection in a group of highly vulnerable individuals - pregnant women - that is associated with a horrible outcome for their babies."
"The real problem is that trying to develop a vaccine that would have to be tested on pregnant women is a practical and ethical nightmare," said Mike Turner, head of infection and immuno-biology at the trust.
The mosquito species that spreads Zika, Aedes aegypti, has been expanding its range over the past few decades. "It loves urban life and has spread across the entire tropical belt of the planet, and of course that belt is expanding as global warming takes effect," said Farrar.
Only extreme measures are likely to contain the Zika threat, said Turner. These could include the use of the banned DDT pesticide.
- Telegraph Group Ltd, AP, Observer