The World Health Organization (WHO) declared the Zika virus an international health emergency on Monday, a rare move prompted by the “explosive” spread of the mosquito-borne virus.

The Zika virus is now in more than 20 countries and territories in the Americas, with no effective vaccine or treatmet yet developed. Although most cases of the infection are mild with very few symptoms, the threat to pregnant women is concerning. The virus is linked to birth defects, specifically microcephaly – a neurological condition where an infant is born with an abnormally small head. At this time, the microcephaly link is "strongly suspected” but not proven.

“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,” said WHO Director-General Margaret Chan at a news conference in geneva. She added that “an international response is needed to minimize the threat in infected countries and reduce risk of international spread.”

Up to 4 million cases of the Zika virus are expected by the end of the year, according to WHO estimates. More than 4,000 cases of microcephaly have been reported in Brazil since October.

First identified in 1947, a recent surge in children born with microcephaly and people with Guillain-Barre syndrome – rare disorder where peripheral nervous system damage causes muscle weakness – has created cause for concern. After facing heavy criticism for their late response to the Ebola outbreak, WHO officials are now taking a much more assertive stance.

“The WHO took a very serious hit to their reputation,” said Dr. Ron Waldman, a George Washington University professor of global health, to The New York Times. “They do have to be mindful of the politics, but they have to get the science right, too. They don’t have much room for slip-ups.”

A state of emergency is declared when the WHO believes more resources and efforts are immediately needed to prevent an outbreak from escalating further. The WHO can also now issue travel advisories and coordinate efforts to combat its spread.