The World Health Organization, which declared the spread of the Zika virus a global health emergency, said it needs $56 million from donors to kickstart the response against it.

WHO doesn't have a lot of money itself to pour into fighting outbreaks and has to raise the cash from member nations and from other donors, including large nonprofits.

Some experts say the expensive undertaking might be too little too late. Global health law expert Lawrence Gostin, also the director of the O'Neill Institute for National and Global Health Law at Georgetown University said the $56 million "is far less than is required to stem the Zika epidemic in the Americas. The urgent need for aggressive mosquito control, surveillance, and research will require major global funding."

Gostin said $4.5 billion was a more realistic figure to fight pandemics.

Zika is carried by the Aedes aegypti and related species of mosquitoes, and WHO has said the best way to quickly fight Zika is to get rid of the pests. Companies are also working on vaccines. Inovio Pharmaceuticals Inc. said Wednesday it had already made an experimental Zika vaccine and tests in mice looked positive.