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Zika: What is a WHO 'Public Health Emergency'?

Aedes aegypti mosquitoes sit in a petri dish at the Fiocruz institute in Recife, Pernambuco state, Brazil,Jan. 27, 2016

The World Health Organization (WHO) declared a global emergency over the explosive spread of the Zika virus, which has been linked to birth defects in the Americas, calling it an "extraordinary event" that poses a public health threat to other parts of the world.

The U.N. agency took the rare step Monday (Feb. 1) despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil, and following a 2013-14 outbreak in French Polynesia.

Reports of a serious neurological condition that can lead to paralysis have also risen in areas where the virus has been reported.

Generally, a disease outbreak must meet two key criteria for the WHO to declare it a "public health emergency of international concern," or PHEIC.

It must first be determined to constitute a public health risk to other countries by spreading internationally and, second, require a coordinated international response.

The organization's director-general is responsible for determining whether an event falls within these categories and for convening an emergency committee.

The committee then advises the director-general on recommended measures to be administered on an emergency basis, known as temporary recommendations.

"After a review of the evidence, the committee advised that the clusters of microcephaly and other neurological complications constitute an extraordinary event and public health threat to other parts of the world," WHO Director-General Dr. Margaret Chan said.

The WHO has only declared a public health emergency three times since its International Health Regulations were enacted in 2007.

The first time was in 2009 with the outbreak of the H1N1 swine flu pandemic. The second was in May 2014 when polio seemed to surge again, threatening the eradication effort.

The third time, in August 2014, came as the Ebola outbreak in West Africa was growing out of control.

That means the Zika declaration will be the fourth PHEIC in history. It is also the first time the WHO has issued such a warning over a mosquito-borne disease.