The alarming infection may be linked to babies being born with underdeveloped brains and has been found in 21 countries in the Caribbean, as well as North and South America.
First detected in monkeys in Africa in 1947, there have been small outbreaks in Africa, Asia and the Pacific. However, in 2015 transmission was detected in Brazil.
The disease is transmitted by Aedes mosquitoes, which are found in all countries in the Americas apart from Canada and Chile. They also spread dengue fever and chikungunya.
The WHO’s regional office, the Pan American Health Organization (PAHO), said: "PAHO anticipates that Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found."
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There is also early evidence of "one case of possible person-to-person sexual transmission".
The director general of the WHO, Margaret Chan, said the outbreak was "extremely worrisome".
Around 80% of infections do not result in symptoms - and even patients who do exhibit symptoms are only likely to suffer from fever and a rash. There is currently no known cure.
However, the biggest concern is the potential impact on babies developing in the womb. There have been around 3,500 reported cases of microcephaly - babies born with tiny brains - in Brazil alone since October.
With no treatment or vaccine available, desperate measures are being taken with Colombia, Ecuador, El Salvador and Jamaica even recommending that women should delay getting pregnant.
With the Olympics taking place in Rio de Janeiro later this year, Brazilian authorities have announced plans to prevent the spread of the virus.
Daily sweeps will take place during the Games, and inspections of Games facilities will begin four months before the Olympics start to prevent mosquito breeding grounds. Fumigation, however, could present health problems for athletes and visitors, and would only be carried out on a case-by-case basis.
The Brazilian health ministry hopes that with the Olympics Games taking place in August mosquitoes will be less prevalent owing to the cooler weather.
A British Olympic Association spokesperson said that its medical team had been liaising with specialists at the London School of Tropical Medicine and would monitor the situation over the coming months.
But fumigation would only be recommended on a case-by-case basis because of concerns for the health of the athletes and visitors.
The PAHO’s advice is to ensure all containers that can hold even small amounts of water should be emptied and cleaned to prevent mosquitoes breeding.
Also, people should protect themselves by using insect repellent, covering up bare skin, and keeping windows and doors closed.