The mosquito-spread Zika virus linked to a spike in birth defects and neurological syndromes is a public health emergency of international concern, the World Health Organization declared Monday.

The declaration followed an emergency meeting in Geneva, in which health experts from around the world reviewed the data on the outbreak blazing through South and Central Americas. In some areas, infection with the Zika virus has been associated with a paralyzing neurological condition, known as Guillain-Barré syndrome, and microcephaly, in which babies are born with severely shrunken and deformed heads and brains.

In Brazil, which reported its first case of Zika last May, the virus has infected an estimated million people and been linked to a 20-fold increase in microcephaly cases. Since the outbreak began, health officials there have reported around 4,000 confirmed and suspected cases of microcephaly, compared to just 147 in 2014.

“The experts agreed that a causal relationship between Zika infection during pregnancy and microcephaly is strongly suspected, though not yet scientifically proven,” Margaret Chan, WHO Director-General, said in a press conference following the meeting. “All agreed on the urgent need to coordinate international efforts to investigate and understand this relationship better.”

WHO and the panel of experts were careful to note that the link to the birth defects and neurological complications were the cause for concern, not the virus itself.

“Zika alone would not be a public health emergency of international concern,” David Heymann, chair of the WHO Emergency Committee said during the conference. It’s only because of the association with microcephay and Guillain-Barré syndrome, he noted. Usually, Zika, a virus related to dengue and discovered decades ago in Uganda, causes mild illness marked by fever, rashes, and aches.

To address the spread of Zika, which has now invaded 25 countries and territories in the Western Hemisphere since last year, WHO has called for more surveillance on microcephaly cases and the viral outbreak, a boost in mosquito population control efforts, and concerted research to study the virus' role in the conditions and develop a vaccine against the virus.

The declaration of an emergency will likely spark new actions and funding from governments and other health agencies, which may look to WHO as an international coordinator.