A disease linked to the Zika virus in Latin America poses a global public health emergency requiring a united response, says the World Health Organization.
Experts are worried that the virus is spreading far and fast, with devastating consequences.
The infection has been linked to cases of microcephaly, in which babies are born with underdeveloped brains.
The WHO alert puts Zika in the same category of concern as Ebola.
It means research and aid will be fast-tracked to tackle the infection.
There have been around 4,000 reported cases of microcephaly in Brazil alone since October.
WHO director general, Margaret Chan called Zika an “extraordinary event” that needed a co-ordinated response.
“I am now declaring that the recent cluster of microcephaly and other neurological abnormalities reported in Latin America following a similar cluster in French Polynesia in 2014 constitutes a public health emergency of international concern.”