On Monday, WHO Director-General Margaret Chan said Zika, which has spread rapidly through parts of Central and South America, represents “an extraordinary event and a public health threat to other parts of the world.”

Labeling the outbreak as a “public health emergency of international concern” is a rare move for WHO. The designation–the highest level alert by WHO–is only made when WHO determines the disease to be a serious international risk, and it is meant to quickly galvanize governments and industries.

This is only the fourth time WHO has declared a public health emergency since 2007, following the enactment of the International Health Regulations. The 2009 H1N1 swine flu outbreak, the 2014 polio surge, and the 2014 Ebola epidemic were all labeled public health emergencies by WHO.

The Zika virus arrived in Brazil last May and spread into 22 other countries and territories around the region. The virus’ spread has been accompanied by a steep increase in babies born with abnormally small heads and brain damage, a condition called microcephaly, and in cases of Guillain-Barre syndrome, an uncommon nervous system disease.

This has raised the alarm among public health officials around the world—and launched programs around the world to study the disease and find treatments and preventions. The decision to declare Zika an urgent public health matter was made by an 18-member advisory panel of experts, dubbed an “emergency committee,” that met Monday in Geneva.

The advisory panel agreed that a causal relationship between Zika and microephaly is “strongly suspected,” though not yet proven. “Even the clusters of microcephaly alone are enough to declare a public health emergency because of its heavy burden” on women, families, and the community, Chan said.

Chan said that case control studies looking at the link between Zika and microcephaly will begin within two weeks.