According to the WHO, an international health emergency is declared when a situation is “serious, unusual or unexpected; carries implications for public health beyond the affected state’s national border; and may require immediate international action.”
The announcement was made after senior WHO officials, joined by representatives of affected countries and experts from around the globe, met behind closed doors in Geneva on Monday to discuss the Zika problem.
The WHO is under pressure to act quickly in the fight against Zika, after admitting it was slow to respond to the recent Ebola outbreak that ravaged parts of West Africa and killed more than 11,000 people.
The Zika virus is transmitted by mosquito and is related to the dengue, yellow fever and West Nile viruses. Infection symptoms are mild and include fever, rashes and joint pain.
The disease is strongly suspected to be linked to microcephaly, a birth defect characterized by incomplete brain development and an unusually small head.
Brazil is the hardest-hit country and sounded the alarm in October, when a rash of microcephaly cases emerged in the country's northeast. Since then, there have been 270 confirmed cases of microcephaly and 3,448 suspected cases.
Worries over Zika have spread beyond the affected areas to Europe and North America, where dozens of cases have been identified among people who traveled to the Caribbean and Central and South America.
There is no vaccine or specific medicine currently available, and treatment is normally focused on relieving symptoms. Research is being done to develop a rapid test to look for antibodies after a patient has recovered from the virus, making it possible to test for immunity.
Only 1 in 5 people infected becomes ill, treatment in hospitals is uncommon, and deaths are rare.
Al Jazeera and wire services